Focus on head shots and keeping arrows flying. If you haven't noticed yet, when you release a shot before your cooldown has recovered, it'll shoot a standard arrow.
Beyond that, prioritizing which enemies to focus on is the next most important thing to do. On both level 4 and 5, the dire wolves and the archers are your biggest threats, so focus on taking them out as quickly as possible.
In terms of upgrades, "Crank to damage" and "Triple Arrow" are the early upgrades to buy, in my opinion. If you can afford the "Repulse Spell", it's the best way to take out the bats (but ultimately the bats are more of a distraction than a threat -- they do very little damage).
And thanks for playing!