Those Wild Monsters aren't joking at all. They're gonna throw EVERYTHING they got to destroy you. Luckily, they don't use any Catapults, but then again, there are some waves wherein they fling in CHAMPIONS, and sometimes they get unfair and they're able to fling all 3 Champions AT ONCE. Luckily, there's this guide which will help you defend your base for this event.
Useful design techniques
Big Dumb Bunch
This design is useful for this event, because the Wild Monsters won't use any Catapults! And this will give you an advantage because you have a huge amount of Concentrated fire.
This design is also useful because it will prevent looters like InvisiBrains (Yes they use this ability) from getting into your precious Resources. You may need to reload your Traps when they attack you using Looters.
Suggested Bunker Combos
All Whirlwind Bandito
The Wild Monster Tribes will send in HUGE wads of Spurtz, more than 40 in later waves. The Whirlwind Banditos will be very effective in taking them out.
This is another useful combination, very useful because the Splash Wormzers move faster than Eye-ras, and they kill the weak Monster bait just by popping out. The Eye-ras will take care of other threats like Teratorns and the flying Fomor.
All Airburst Eye-ra
You'll need to have them level 6 with full airburst when you plan to use them. This is effective in the late waves of the event, where they send in a whole bunch of level 6 D.A.V.E.s and Champions. Again, using Eye-ra is more effective in the later waves, rendering the healing Zafreetis useless. Note that you may need to refill for every wave.