Use the Grenade Spiker (the 6 barreled one) if you have a decent aim. Combine with the Acid Bomb Thrower and they are brilliant. Camp on the middle level of the "E" on the left side of Medium Laboratory.
They overran me at wave 164. Stupid turrets cannot get decent lock-on to a flying one. Had to stand by exit and blow them them away with the Machinegun. Works like a treat, and they dropped like $700 a death. Simple, easy money.
Good news! I found the level limit! you get asked a question at wave 70. It says: You have defeated the ranks of the shadow horde. You can now choose to fight on against the endless monsters to your last breath or choose another battle.
To get this, I had 4 Cash Guns (Fully Upgraded, they give $10 A HIT), 5 Slowing Pads (Fully Upgraded) and the Machinegun; all stats maxed out (Level 20 damage, 7 rate of fire, 10 capacity and 6 reload speed. Upgrade rate of fire first, keeping damage t a decent level. Capacity is very important in the last 40 levels.
Second KillingDude! You just completed the Second Killing achievement in Bunny Invasion: Easter Special and won the Second Killing badge and 30 points!
Got all the trophies to.
 "The Idle RPG")
 "Thing-Thing Arena 3")
 "UPGRADE COMPLETE!")
 "Crazy Flasher 4")
 "Upgrade Complete")
 "Eternal Red")
 "Ultimate Tactics")
 "The Pen Is Mightier Than The Sword")
 "The Torture Game 2")
 "Bunny Invasion: Easter Special")