I got the same problem dude. All you can do is make sure you have a lot of sun and let the lawnmowers kill the giga-gargantuars (it seems to be the only thing that kills em). After that, try to make a solid defense and it might keep you going for a few rounds. Here's a tip: if you use the strategy that involves gloomshrooms in the water, put some garlic in the outer lanes to get all zombies in the reach of the gloomshrooms. I really hope they fix this bug anytime soon.
