The only real obstacle is the Demons,but i use a method i found out quite awhile ago.You can lay a bunch of barrels that explode and save like 20 of em and when they ignite.Start laying barrrels and u will have like a explosion as your running.Quite useful.
My onyl sggestion is to not upgrade bombs.ALways get ready to use your best unit frist. in the begginning of a battle.And at the end gl. 5/5 should make a ww2 game like with them in a field or forest with planes and such idk.