Since it boosts all zombies on the field, make sure you have as many zombies on the field to maximize the benefit from this trick. Also, this trick is extremely useful for every Hearty deck, since it is an effective way to boost the strength and health of zombies. With Z-Mech, it can aid dancing decks, especially with Aerobics Instructor. However, this trick is expensive, so you should prepare cheaper boosting cards as well, or try to get more brains to play it earlier, primarily as Rustbolt.
Against
Just like all boosting tricks, you really can't counter this. However, you can predict it. If your opponent does nothing on the 5th turn, you can expect Monster Mash to be played. In this case, try to Bounce, Freeze, or destroy zombies so that they are fewer zombies that can be boosted. If this cannot be done, but if you are playing as a Guardian hero, you can play Template:PvZHLink, should most zombies go over 4, though you need to be careful that your plants do not go over 4 as well.