This is not a cheat, its a Guide.

When you and/or a Partner(s) reach wave 30 do the following:

1st of all you should load up the map Sandbar. There will be two Boss characters (every 10th wave is a Boss wave).

The Boss will be a monster with 8 arms. He is Lambert (glowing bright yellow).

When you start there is a cave across the river (it looks like some hippies had a party there and they got wasted - there is empty beer bottles, a camp fire and a rug they sat on). In the cave there is ammo...

Okay now buy the Command Post. Buy all barriers and the turrets (if you have enough cash).

These Boss characters have a glowing yellow ball on their stomach. But (and this is important) this boss covers it with his 8 arms and you will not see it.

He will open up his arms eventually and then you just shoot the crap out of that glowing yellow ball!

He will then close it up again and run towards you! Dodge him and run away. He will then open up his arms again and then you shoot that yellow ball! Repeat this at least 4 times and you will be victorious.