Use ice and darkhole (if you don't lag with these) towers.
There's no need to boost the towers, all you need to focus on is to hit the enemies.
As a warlock, just spamming your skills all around is efficient. Fire emblem can slow down enemies with its frozen spikes (I believe it stacks with ice tower's freezing) and blizzard, which will completely lock the monsters.
Success is guaranteed if you are followed by a paladin and/or other fellow warlocks.
Also, considering trying in a few more levels (like 5 or above) is a great choice, since you will unlock more skills to throw to these walking dummies