Grass types are immune to powder moves

src/data/move.ts

@@ -161,6 +161,16 @@ export default class Move {
     return false;
   }
 
+  isTypeImmune(type: Type): boolean {
+    switch (type) {
+      case Type.GRASS:
+        if (this.hasFlag(MoveFlags.POWDER_MOVE))
+          return true;
+        break;
+    }
+    return false;
+  }
+
   condition(condition: MoveCondition | MoveConditionFunc): this {
     if (typeof condition === 'function')
       condition = new MoveCondition(condition as MoveConditionFunc);

src/field/pokemon.ts

@@ -1044,6 +1044,8 @@ export default abstract class Pokemon extends Phaser.GameObjects.Container {
       : 1);
     if (typeless)
       typeMultiplier.value = 1;
+    if (this.getTypes(true, true).find(t => move.isTypeImmune(t)))
+      typeMultiplier.value = 0;
 
     switch (moveCategory) {
       case MoveCategory.PHYSICAL: