Dorestes, I beat level 16 by sending multiple titan units under the Elven Race. Since the titan unit lifts enemy units off the ground, it takes less damage from everyone but the frog. The Frog I killed just using lots of arrows, titans, and a mix of archers. The titan took the big damage while the archers were able to deal out damage at a distance. Hope this helps. Anyone have advice for levels 17 and 18?