Adding what works to the game even though it's not the best.

src/battle-scene.ts

@@ -1233,13 +1233,40 @@ export default class BattleScene extends SceneBase {
       newDouble = !!double;
     }
 
-    if (Overrides.BATTLE_TYPE_OVERRIDE === "double") {
+    // Disable double battles on Endless/Endless Spliced Wave 50x boss battles (Introduced 1.2.0)
+    if (this.gameMode.isEndlessBoss(newWaveIndex)) {
+      newDouble = false;
+    }
+
+    if (!isNullOrUndefined(Overrides.BATTLE_TYPE_OVERRIDE)) {
+      let doubleOverrideForWave: "single" | "double" | null = null;
+
+      switch (Overrides.BATTLE_TYPE_OVERRIDE) {
+        case "double":
+          doubleOverrideForWave = "double";
+          break;
+        case "single":
+          doubleOverrideForWave = "single";
+          break;
+        case "even-doubles":
+          doubleOverrideForWave = (newWaveIndex % 2) ? "single" : "double";
+          break;
+        case "odd-doubles":
+          doubleOverrideForWave = (newWaveIndex % 2) ? "double" : "single";
+          break;
+      }
+
+      if (doubleOverrideForWave === "double") {
         newDouble = true;
       }
-    /* Override battles into single only if not fighting with trainers */
+      /**
-    if (newBattleType !== BattleType.TRAINER && Overrides.BATTLE_TYPE_OVERRIDE === "single") {
+       * Override battles into single only if not fighting with trainers.
+       * @see {@link https://github.com/pagefaultgames/pokerogue/issues/1948 | GitHub Issue #1948}
+       */
+      if (newBattleType !== BattleType.TRAINER && doubleOverrideForWave === "single") {
         newDouble = false;
       }
+    }
 
     const lastBattle = this.currentBattle;
 
@@ -1397,10 +1424,19 @@ export default class BattleScene extends SceneBase {
       case Species.PALDEA_TAUROS:
         return Utils.randSeedInt(species.forms.length);
       case Species.PIKACHU:
+        if (this.currentBattle?.battleType === BattleType.TRAINER && this.currentBattle?.waveIndex < 30) {
+          return 0; // Ban Cosplay and Partner Pika from Trainers before wave 30
+        }
         return Utils.randSeedInt(8);
       case Species.EEVEE:
+        if (this.currentBattle?.battleType === BattleType.TRAINER && this.currentBattle?.waveIndex < 30) {
+          return 0; // No Partner Eevee for Wave 12 Preschoolers
+        }
         return Utils.randSeedInt(2);
       case Species.GRENINJA:
+        if (this.currentBattle?.battleType === BattleType.TRAINER) {
+          return 0; // Don't give trainers Battle Bond Greninja
+        }
         return Utils.randSeedInt(2);
       case Species.ZYGARDE:
         return Utils.randSeedInt(4);
@@ -2584,18 +2620,10 @@ export default class BattleScene extends SceneBase {
     return new Promise(resolve => {
       const source = itemModifier.pokemonId ? itemModifier.getPokemon(target.scene) : null;
       const cancelled = new Utils.BooleanHolder(false);
-      if (source && source.isPlayer() !== target.isPlayer()) {
+      Utils.executeIf(!!source && source.isPlayer() !== target.isPlayer(), () => applyAbAttrs(BlockItemTheftAbAttr, source! /* checked in condition*/, cancelled)).then(() => {
-        //Check for abilities like Sticky Hold that prevent item transfer between player and opponent
-        applyAbAttrs(BlockItemTheftAbAttr, source, cancelled).then(() => {
-          // Check to prevent player-to-opponent/opponent-to-player MBH transfer
-          if (!cancelled.value) {
-            cancelled.value = itemModifier instanceof TurnHeldItemTransferModifier;
-          }
         if (cancelled.value) {
           return resolve(false);
         }
-        });
-      }
         const newItemModifier = itemModifier.clone() as PokemonHeldItemModifier;
         newItemModifier.pokemonId = target.id;
         const matchingModifier = target.scene.findModifier(m => m instanceof PokemonHeldItemModifier
@@ -2647,6 +2675,7 @@ export default class BattleScene extends SceneBase {
         }
         resolve(false);
       });
+    });
   }
 
   removePartyMemberModifiers(partyMemberIndex: integer): Promise<void> {

src/phases/encounter-phase.ts

@@ -12,7 +12,7 @@ import { getRandomWeatherType } from "#app/data/weather";
 import { EncounterPhaseEvent } from "#app/events/battle-scene";
 import Pokemon, { FieldPosition } from "#app/field/pokemon";
 import { getPokemonNameWithAffix } from "#app/messages";
-import { BoostBugSpawnModifier, IvScannerModifier } from "#app/modifier/modifier";
+import { BoostBugSpawnModifier, IvScannerModifier, TurnHeldItemTransferModifier } from "#app/modifier/modifier";
 import { ModifierPoolType, regenerateModifierPoolThresholds } from "#app/modifier/modifier-type";
 import Overrides from "#app/overrides";
 import { BattlePhase } from "#app/phases/battle-phase";
@@ -438,6 +438,14 @@ export class EncounterPhase extends BattlePhase {
       if (enemyPokemon.isShiny()) {
         this.scene.unshiftPhase(new ShinySparklePhase(this.scene, BattlerIndex.ENEMY + e));
       }
+      if (enemyPokemon.species.speciesId === Species.ETERNATUS && this.scene.gameMode.isEndlessMajorBoss(this.scene.currentBattle.waveIndex)) {
+        const enemyMBH = this.scene.findModifier(m => m instanceof TurnHeldItemTransferModifier, false) as TurnHeldItemTransferModifier;
+        if (enemyMBH) {
+          this.scene.removeModifier(enemyMBH, true);
+          enemyMBH.setTransferrableFalse();
+          this.scene.addEnemyModifier(enemyMBH);
+        }
+      }
     });
 
     if (![ BattleType.TRAINER, BattleType.MYSTERY_ENCOUNTER ].includes(this.scene.currentBattle.battleType)) {