Build 1 first, then 2, then 3. Don't upgrade unless you have extra mana, and you're upgrading for boss. There are a few ways to finish.
-Boost monk at #3 for boss
-Stay apprentice and sell all of #1 at the end of wave 5 and build at #4 before boss
-Or play DPS for wave 5 and help kill the boss if your DSTs aren't killing him before he moves out of range.