Swap fireballs for more Mortars if you keep failing during waves. Switching to jester during waves and rolling dmg might help you get to boss. During boss fight the enemies are a lot easier for the towers to kill.
1. Build on top of the Gargoile statue and down to the edge of the building
2. Top of stairs is where you want your stack
3. Build the BB from bottom of the rock to to top as far as you can get it and your Mortar stack to the right of the rock.
First Wave
Auras, Beams, Golem
Fire balls + Seed bomb North
Upgrade North BB to 2 stars
During wave upgrade quickly South BB all the way and move to North for the rest of the wave
Second Wave
Fire balls + Seed bomb South
Bulwarks
During wave start upgrading from South
Boss Wave
Log your 4 characters of choice and wait for towers to kill the boss
Or stand on 2 and dps with a long range weapon (you might have to move from towers from time to time for them to heal up)