Clear the RoomG 5

Clear the Room G5:
Kill 250 undead with the Katana Wave ability

This achievement is a little confusing at first, since Katana Wave isn't actually the official name of anything in game. Katana Wave in this sense actually is referring to the "Troop Trample" power that is unlockable after level 40. This move is great and will be your main move for getting through waves 40-50 as read in "This land is Safe", so make sure you upgrade it often and spam it as much as you can. Killing 250 zombies will simply come with natural progression of the waves.