[MunchkinMan]

Cards are resolved as they are played. The helper's card hit the table first, his counts first. There are no kind of crazy timing rules, or a stack in Munchkin, and the "2.6 second" rule doesn't apply to every single action. The only time that you have to give people a reasonable amount of time to do something is when it comes to declaring victory in combat.

So, the situation you described goes like this: Player A is in combat with two Monster, one of which is Undead. Player A gets Player B to help, and he immediately plays a card which dismisses the Undead Monster. The munchkin side is now winning, so now they have to give everyone else a reasonable amount of time to find a way to stop them. Removing the helper is clearly not a worthwhile exercise at this point, so you might as well hold on to your Cytillesh Potion, unless you, or someone else, can wander in a Monster that they can't beat, or can play enough Monster Enhancers to make the Potted Plant that much harder to beat.