EDIT: this is for Frontiers!

Any tips on beating the final boss? I reached his 3rd incarnation, but by that time he's destroyed all my towers but 3 and sends hordes of shadows that just overrun me.

I read that you should save money to be able to rebuild, but I saved 5000 (!) gold when he appeared, and still that wasn't enough. Rebuilding the towers he destroyed on second incarnation cost me around 4000, and by the third incarnation I was broke again. I honestly doubt I could save more. And this was with assassins on both side passages stealing money.

Beat it!

Here is a tip in case someone has the same problem. Don't upgrade all the destroyed towers. If the boss destroys a tower build a level 1 archer in its place. Since the boss destroys towers randomly, there is a good chance he'll destroy the weak ones. Make sure all spots have a tower. If you feel you have too little defenses, then only upgrade one to the max. Crossbow seem to be best and are cheapest.

And you need money saved. I had 5800 before the boss. Basically avoid spending money the last 2 waves.