Tower tactics.

When playing as Gremlins without a cart, you need to be quite precise, trying to shoot as many direct shots as possible. Nagas in the Castle are extremely rare, as only Maretti knows how to play with un-upgraded Gremlins; the vast majority upgrade them.
By the way, I have always advocated for using Kirr when playing as Tower. Many Tower heroes have useful magic (which can be passed on!), and this magic can help in breaking through defenses.