diff --git a/src/@types/stat-types.ts b/src/@types/stat-types.ts
index bcba054383e..9b9ec8f5fea 100644
--- a/src/@types/stat-types.ts
+++ b/src/@types/stat-types.ts
@@ -19,7 +19,7 @@ export type PermanentStatTuple = Head<Head<StatTuple>>;
 /** Tuple containing all {@linkcode BattleStat}s of a Pokemon. */
 export type BattleStatTuple = Tail<PermanentStatTuple>;
 
-// Since typescript lacks integer range unions, we have to use THIS abomination to strongly type an IV
+/** Integer literal union containing all numbers from 0-31 inclusive; used to strongly type Pokemon IVs.  */
 export type IVType =
   | 0
   | 1
@@ -56,4 +56,5 @@ export type IVType =
 
 type toIvTuple<T extends [...any]> = { [k in keyof T]: IVType };
 
+/** A 6-length tuple of integers in the range [0-31]; used to strongly type Pokemon IVs. */
 export type IVTuple = toIvTuple<PermanentStatTuple>;
diff --git a/src/@types/utility-types/global-augments.ts b/src/@types/utility-types/global-augments.ts
new file mode 100644
index 00000000000..c0a74b42d9b
--- /dev/null
+++ b/src/@types/utility-types/global-augments.ts
@@ -0,0 +1,9 @@
+export {};
+
+declare global {
+  // Array global augments to allow semi-easy working with tuples...???
+  interface Array<T> {
+    map<U>(callbackfn: (value: T, index: number, array: this) => U, thisArg?: any): { [K in keyof this]: U };
+    slice(start: 0): { [K in keyof this]: T };
+  }
+}
diff --git a/src/battle-scene.ts b/src/battle-scene.ts
index 0adce63134f..a2ebd8b9fd4 100644
--- a/src/battle-scene.ts
+++ b/src/battle-scene.ts
@@ -160,7 +160,7 @@ import { timedEventManager } from "./global-event-manager";
 import { starterColors } from "./global-vars/starter-colors";
 import { startingWave } from "./starting-wave";
 import { PhaseManager } from "./phase-manager";
-import type { IVTuple } from "#app/@types/stat-types";
+import type { IVTuple, IVType } from "#app/@types/stat-types";
 
 const DEBUG_RNG = false;
 
@@ -169,7 +169,7 @@ const OPP_IVS_OVERRIDE_VALIDATED: IVTuple | null =
     ? null
     : Array.isArray(Overrides.OPP_IVS_OVERRIDE)
       ? Overrides.OPP_IVS_OVERRIDE
-      : new Array(6).fill(Overrides.OPP_IVS_OVERRIDE);
+      : (new Array(6).fill(Overrides.OPP_IVS_OVERRIDE) as IVTuple);
 
 export interface PokeballCounts {
   [pb: string]: number;
@@ -910,7 +910,7 @@ export default class BattleScene extends SceneBase {
     gender?: Gender,
     shiny?: boolean,
     variant?: Variant,
-    ivs?: number[],
+    ivs?: IVTuple,
     nature?: Nature,
     dataSource?: Pokemon | PokemonData,
     postProcess?: (playerPokemon: PlayerPokemon) => void,
@@ -958,23 +958,20 @@ export default class BattleScene extends SceneBase {
     }
 
     if (boss && !dataSource) {
+      // Generate a 2nd higher roll and linearly interpolate between the two.
       const secondaryIvs = getIvsFromId(randSeedInt(4294967296));
 
-      for (let s = 0; s < pokemon.ivs.length; s++) {
-        pokemon.ivs[s] = Math.round(
-          Phaser.Math.Linear(
-            Math.min(pokemon.ivs[s], secondaryIvs[s]),
-            Math.max(pokemon.ivs[s], secondaryIvs[s]),
-            0.75,
-          ),
-        );
-      }
+      pokemon.ivs = pokemon.ivs.map(
+        (iv, i) =>
+          Math.round(Phaser.Math.Linear(Math.min(iv, secondaryIvs[i]), Math.max(iv, secondaryIvs[i]), 0.75)) as IVType, // ts doesn't know the number will be between 0-31
+      );
     }
+
     if (postProcess) {
       postProcess(pokemon);
     }
 
-    if (OPP_IVS_OVERRIDE_VALIDATED) {
+    if (OPP_IVS_OVERRIDE_VALIDATED !== null) {
       pokemon.ivs = OPP_IVS_OVERRIDE_VALIDATED;
     }
     pokemon.init();
@@ -2084,7 +2081,7 @@ export default class BattleScene extends SceneBase {
     let scoreIncrease =
       enemy.getSpeciesForm().getBaseExp() *
       (enemy.level / this.getMaxExpLevel()) *
-      ((enemy.ivs.reduce((iv: number, total: number) => (total += iv), 0) / 93) * 0.2 + 0.8);
+      ((enemy.ivs.reduce((total, iv) => total + iv, 0) / 93) * 0.2 + 0.8);
     this.findModifiers(m => m instanceof PokemonHeldItemModifier && m.pokemonId === enemy.id, false).map(
       m => (scoreIncrease *= (m as PokemonHeldItemModifier).getScoreMultiplier()),
     );
diff --git a/src/data/egg.ts b/src/data/egg.ts
index 67cdb7b1344..15679fc7d02 100644
--- a/src/data/egg.ts
+++ b/src/data/egg.ts
@@ -38,6 +38,7 @@ import {
   HATCH_WAVES_LEGENDARY_EGG,
 } from "#app/data/balance/rates";
 import { speciesEggTiers } from "#app/data/balance/species-egg-tiers";
+import type { IVType } from "#app/@types/stat-types";
 
 export const EGG_SEED = 1073741824;
 
@@ -270,9 +271,9 @@ export class Egg {
 
       const secondaryIvs = getIvsFromId(randSeedInt(4294967295));
 
-      for (let s = 0; s < ret.ivs.length; s++) {
-        ret.ivs[s] = Math.max(ret.ivs[s], secondaryIvs[s]);
-      }
+      ret.ivs = ret.ivs.map(
+        (iv, i) => Math.max(iv, secondaryIvs[i]) as IVType, // ts doesn't know the number will be between 0-31
+      );
     };
 
     ret = ret!; // Tell TS compiler it's defined now
diff --git a/src/data/mystery-encounters/encounters/weird-dream-encounter.ts b/src/data/mystery-encounters/encounters/weird-dream-encounter.ts
index 83e876d1aa8..c930c86ebb4 100644
--- a/src/data/mystery-encounters/encounters/weird-dream-encounter.ts
+++ b/src/data/mystery-encounters/encounters/weird-dream-encounter.ts
@@ -511,7 +511,7 @@ async function postProcessTransformedPokemon(
       const hiddenAbilityChance = new NumberHolder(256);
       globalScene.applyModifiers(HiddenAbilityRateBoosterModifier, true, hiddenAbilityChance);
 
-      const hasHiddenAbility = !randSeedInt(hiddenAbilityChance.value);
+      const hasHiddenAbility = randSeedInt(hiddenAbilityChance.value) === 0;
 
       if (hasHiddenAbility) {
         newPokemon.abilityIndex = hiddenIndex;
diff --git a/src/data/mystery-encounters/utils/encounter-phase-utils.ts b/src/data/mystery-encounters/utils/encounter-phase-utils.ts
index e2b92230985..52bd49a281a 100644
--- a/src/data/mystery-encounters/utils/encounter-phase-utils.ts
+++ b/src/data/mystery-encounters/utils/encounter-phase-utils.ts
@@ -54,6 +54,7 @@ import { PokemonType } from "#enums/pokemon-type";
 import { getNatureName } from "#app/data/nature";
 import { getPokemonNameWithAffix } from "#app/messages";
 import { timedEventManager } from "#app/global-event-manager";
+import type { IVTuple } from "#app/@types/stat-types";
 
 /**
  * Animates exclamation sprite over trainer's head at start of encounter
@@ -96,7 +97,7 @@ export interface EnemyPokemonConfig {
   passive?: boolean;
   moveSet?: MoveId[];
   nature?: Nature;
-  ivs?: [number, number, number, number, number, number];
+  ivs?: IVTuple;
   shiny?: boolean;
   /** Is only checked if Pokemon is shiny */
   variant?: Variant;
diff --git a/src/field/pokemon.ts b/src/field/pokemon.ts
index ddd469281fc..22077d958cf 100644
--- a/src/field/pokemon.ts
+++ b/src/field/pokemon.ts
@@ -6089,12 +6089,12 @@ export class EnemyPokemon extends Pokemon {
 
       if (this.hasTrainer() && globalScene.currentBattle) {
         const { waveIndex } = globalScene.currentBattle;
-        const ivs: IVTuple = Array.from(
+        const ivs = Array.from(
           {
             length: 6,
           },
           () => randSeedIntRange(Math.floor(waveIndex / 10), 31),
-        );
+        ) as IVTuple;
         this.ivs = ivs;
       }
     }
diff --git a/src/phases/encounter-phase.ts b/src/phases/encounter-phase.ts
index f2c23384627..d56a06918af 100644
--- a/src/phases/encounter-phase.ts
+++ b/src/phases/encounter-phase.ts
@@ -121,7 +121,8 @@ export class EncounterPhase extends BattlePhase {
             !!globalScene.getEncounterBossSegments(battle.waveIndex, level, enemySpecies),
           );
           if (globalScene.currentBattle.battleSpec === BattleSpec.FINAL_BOSS) {
-            battle.enemyParty[e].ivs = new Array(6).fill(31);
+            // max IVs for eternatus
+            battle.enemyParty[e].ivs = [31, 31, 31, 31, 31, 31];
           }
           globalScene
             .getPlayerParty()
diff --git a/src/system/game-data.ts b/src/system/game-data.ts
index 00b46b9e5f4..eb47fa33c2d 100644
--- a/src/system/game-data.ts
+++ b/src/system/game-data.ts
@@ -68,6 +68,7 @@ import { DexAttr } from "#enums/dex-attr";
 import { AbilityAttr } from "#enums/ability-attr";
 import { defaultStarterSpecies, saveKey } from "#app/constants";
 import { encrypt, decrypt } from "#app/utils/data";
+import type { IVTuple } from "#app/@types/stat-types";
 
 function getDataTypeKey(dataType: GameDataType, slotId = 0): string {
   switch (dataType) {
@@ -1945,7 +1946,13 @@ export class GameData {
     _unlockSpeciesNature(species.speciesId);
   }
 
-  updateSpeciesDexIvs(speciesId: SpeciesId, ivs: number[]): void {
+  /**
+   * When a pokemon is caught or added, maximize its lowest pre-evolution's {@linkcode DexData}
+   * with the IVs of the newly caught pokemon.
+   * @param speciesId - The {@linkcode SpeciesId} to update dex data for.
+   * @param ivs - The {@linkcode IVTuple | IVs} of the caught pokemon.
+   */
+  updateSpeciesDexIvs(speciesId: SpeciesId, ivs: IVTuple): void {
     let dexEntry: DexEntry;
     do {
       dexEntry = globalScene.gameData.dexData[speciesId];
@@ -1955,7 +1962,7 @@ export class GameData {
           dexIvs[i] = ivs[i];
         }
       }
-      if (dexIvs.filter(iv => iv === 31).length === 6) {
+      if (dexIvs.every(iv => iv === 31)) {
         globalScene.validateAchv(achvs.PERFECT_IVS);
       }
     } while (pokemonPrevolutions.hasOwnProperty(speciesId) && (speciesId = pokemonPrevolutions[speciesId]));
diff --git a/src/system/pokemon-data.ts b/src/system/pokemon-data.ts
index 7571f0cc82f..5b9a1a8c0f6 100644
--- a/src/system/pokemon-data.ts
+++ b/src/system/pokemon-data.ts
@@ -14,6 +14,7 @@ import type { MoveId } from "#enums/move-id";
 import type { SpeciesId } from "#enums/species-id";
 import { CustomPokemonData } from "#app/data/custom-pokemon-data";
 import type { PokemonType } from "#enums/pokemon-type";
+import type { IVTuple, PermanentStatTuple } from "#app/@types/stat-types";
 
 export default class PokemonData {
   public id: number;
@@ -31,8 +32,8 @@ export default class PokemonData {
   public levelExp: number;
   public gender: Gender;
   public hp: number;
-  public stats: number[];
-  public ivs: number[];
+  public stats: PermanentStatTuple;
+  public ivs: IVTuple;
   public nature: Nature;
   public moveset: PokemonMove[];
   public status: Status | null;
diff --git a/src/utils/common.ts b/src/utils/common.ts
index f6036ed981d..b3ca840a88c 100644
--- a/src/utils/common.ts
+++ b/src/utils/common.ts
@@ -3,7 +3,7 @@ import { MoveId } from "#enums/move-id";
 import i18next from "i18next";
 import { pokerogueApi } from "#app/plugins/api/pokerogue-api";
 import type { Variant } from "#app/sprites/variant";
-import type { IVTuple } from "#app/@types/stat-types";
+import type { IVTuple, IVType } from "#app/@types/stat-types";
 
 export type nil = null | undefined;
 
@@ -88,6 +88,8 @@ export function randInt(range: number, min = 0): number {
   return Math.floor(Math.random() * range) + min;
 }
 
+export function randSeedInt(range: 31, min?: IVType): IVType;
+export function randSeedInt(range: number, min?: number): number;
 /**
  * Generate a random integer using the global seed, or the current battle's seed if called via `Battle.randSeedInt`
  * @param range - How large of a range of random numbers to choose from. If {@linkcode range} <= 1, returns {@linkcode min}
@@ -191,7 +193,7 @@ export function getIvsFromId(id: number): IVTuple {
     (id & 0x00007c00) >>> 10,
     (id & 0x000003e0) >>> 5,
     id & 0x0000001f,
-  ];
+  ] as IVTuple; // TS doesn't know the numbers are between 0-31
 }
 
 export function formatLargeNumber(count: number, threshold: number): string {
diff --git a/test/moves/fusion_flare_bolt.test.ts b/test/moves/fusion_flare_bolt.test.ts
index f10ede8717c..f25741ae76f 100644
--- a/test/moves/fusion_flare_bolt.test.ts
+++ b/test/moves/fusion_flare_bolt.test.ts
@@ -11,6 +11,7 @@ import { SpeciesId } from "#enums/species-id";
 import GameManager from "#test/testUtils/gameManager";
 import Phaser from "phaser";
 import { afterEach, beforeAll, beforeEach, describe, expect, it, vi } from "vitest";
+import type { PermanentStatTuple } from "#app/@types/stat-types";
 
 describe("Moves - Fusion Flare and Fusion Bolt", () => {
   let phaserGame: Phaser.Game;
@@ -156,6 +157,7 @@ describe("Moves - Fusion Flare and Fusion Bolt", () => {
     expect(fusionFlare.calculateBattlePower).toHaveLastReturnedWith(200);
   }, 20000);
 
+  // TODO: Clean these tests up
   it("FUSION_FLARE and FUSION_BOLT alternating throughout turn should double power of subsequent moves", async () => {
     game.override.enemyMoveset(fusionFlare.id);
     await game.classicMode.startBattle([SpeciesId.ZEKROM, SpeciesId.ZEKROM]);
@@ -168,7 +170,10 @@ describe("Moves - Fusion Flare and Fusion Bolt", () => {
     game.scene.clearEnemyModifiers();
 
     // Mock stats by replacing entries in copy with desired values for specific stats
-    const stats = {
+    const stats: {
+      enemy: [PermanentStatTuple, PermanentStatTuple];
+      player: [PermanentStatTuple, PermanentStatTuple];
+    } = {
       enemy: [[...enemyParty[0].stats], [...enemyParty[1].stats]],
       player: [[...party[0].stats], [...party[1].stats]],
     };
@@ -222,7 +227,10 @@ describe("Moves - Fusion Flare and Fusion Bolt", () => {
     game.scene.clearEnemyModifiers();
 
     // Mock stats by replacing entries in copy with desired values for specific stats
-    const stats = {
+    const stats: {
+      enemy: [PermanentStatTuple, PermanentStatTuple];
+      player: [PermanentStatTuple, PermanentStatTuple];
+    } = {
       enemy: [[...enemyParty[0].stats], [...enemyParty[1].stats]],
       player: [[...party[0].stats], [...party[1].stats]],
     };