Unlocks within RUINS OF THE ANCIENT KINGDOM
The final battle consists of fighting 4 tiers of monsters, each with one really big monster and lots of respawning ones.
The first one is the giant wolf. This one shouldn't give you any problems.
The second one is the giant fire warthog, with lots of little fire warthogs running around and also tree guys that throw spears. My best piece of advice would be to just keep running around and using charged up ice shots on the big warthog. Once you kill him, everyone one else dies, so concentrate on him. And stay away from the fire trails that they all leave because more than likely, it will kill you very very fast.
Third fight is the giant harpy again. Shouldn't be any problem.
The last fight is with the stone guardian. Here's a huge tip to make this fight a lot easier. Kill everything on the way up normally and when you get to the platform right before the guardian, stay there. His rocks can't hit you on this platform. All I did was constantly spam the earth charged attack. I didn't even get hit once doing this method. Just keep charging up the earth attack and shooting. Make sure it's fully leveled up too. After he's dead, your done!
Note: These boss fights are also a great way to grind out any kills you have left for achievements, as well as make some coin. Once you've beaten the game, you will be back in the level select area. Set the difficulty to easy and play the last level again. Once you get to the final boss battles, the enemies pretty much respawn infinitely.