diff --git a/src/data/ability.ts b/src/data/ability.ts
index a698831a1eb0..8a7365b2c28a 100644
--- a/src/data/ability.ts
+++ b/src/data/ability.ts
@@ -519,6 +519,7 @@ export class FullHpResistTypeAbAttr extends PreDefendAbAttr {
 
     if (pokemon.isFullHp() && typeMultiplier.value > 0.5) {
       typeMultiplier.value = 0.5;
+      pokemon.turnData.moveEffectiveness = 0.5;
       return true;
     }
     return false;
diff --git a/src/field/pokemon.ts b/src/field/pokemon.ts
index 6afc0770a7fe..22df5d3a6c39 100644
--- a/src/field/pokemon.ts
+++ b/src/field/pokemon.ts
@@ -1538,6 +1538,10 @@ export default abstract class Pokemon extends Phaser.GameObjects.Container {
    * @returns The type damage multiplier, indicating the effectiveness of the move
    */
   getMoveEffectiveness(source: Pokemon, move: Move, ignoreAbility: boolean = false, simulated: boolean = true, cancelled?: Utils.BooleanHolder): TypeDamageMultiplier {
+    if (!Utils.isNullOrUndefined(this.turnData?.moveEffectiveness)) {
+      return this.turnData?.moveEffectiveness;
+    }
+
     if (move.hasAttr(TypelessAttr)) {
       return 1;
     }
@@ -5019,6 +5023,7 @@ export class PokemonTurnData {
   public order: number;
   public statStagesIncreased: boolean = false;
   public statStagesDecreased: boolean = false;
+  public moveEffectiveness: TypeDamageMultiplier | null = null;
 }
 
 export enum AiType {
diff --git a/src/phases/move-effect-phase.ts b/src/phases/move-effect-phase.ts
index 263a576c4f08..ca1fb87654ff 100644
--- a/src/phases/move-effect-phase.ts
+++ b/src/phases/move-effect-phase.ts
@@ -354,12 +354,14 @@ export class MoveEffectPhase extends PokemonPhase {
       } else {
         // Queue message for number of hits made by multi-move
         // If multi-hit attack only hits once, still want to render a message
-        const hitsTotal = user.turnData.hitCount! - Math.max(user.turnData.hitsLeft!, 0); // TODO: are those bangs correct?
+        const hitsTotal = user.turnData.hitCount - Math.max(user.turnData.hitsLeft, 0);
         if (hitsTotal > 1 || (user.turnData.hitsLeft && user.turnData.hitsLeft > 0)) {
           // If there are multiple hits, or if there are hits of the multi-hit move left
           this.scene.queueMessage(i18next.t("battle:attackHitsCount", { count: hitsTotal }));
         }
         this.scene.applyModifiers(HitHealModifier, this.player, user);
+        // Clear all cached move effectiveness values among targets
+        this.getTargets().forEach((target) => target.turnData.moveEffectiveness = null);
       }
     }
 
diff --git a/src/test/abilities/tera_shell.test.ts b/src/test/abilities/tera_shell.test.ts
index 13df49136ca7..9995f7c34c3e 100644
--- a/src/test/abilities/tera_shell.test.ts
+++ b/src/test/abilities/tera_shell.test.ts
@@ -1,3 +1,4 @@
+import { BattlerIndex } from "#app/battle";
 import { Abilities } from "#app/enums/abilities";
 import { Moves } from "#app/enums/moves";
 import { Species } from "#app/enums/species";
@@ -106,4 +107,26 @@ describe("Abilities - Tera Shell", () => {
       expect(playerPokemon.hp).toBe(playerPokemon.getMaxHp() - 40);
     }
   );
+
+  it(
+    "should change the effectiveness of all strikes of a multi-strike move",
+    async () => {
+      game.override.enemyMoveset([Moves.DOUBLE_HIT]);
+
+      await game.classicMode.startBattle([Species.SNORLAX]);
+
+      const playerPokemon = game.scene.getPlayerPokemon()!;
+      vi.spyOn(playerPokemon, "apply");
+
+      game.move.select(Moves.SPLASH);
+
+      await game.setTurnOrder([BattlerIndex.ENEMY, BattlerIndex.PLAYER]);
+      await game.move.forceHit();
+      for (let i = 0; i < 2; i++) {
+        await game.phaseInterceptor.to("MoveEffectPhase");
+        expect(playerPokemon.apply).toHaveLastReturnedWith(HitResult.NOT_VERY_EFFECTIVE);
+      }
+      expect(playerPokemon.apply).toHaveReturnedTimes(2);
+    }
+  );
 });