My suggestion is - upon start of the level head left (left area is a bit wider then the right one).
1) Start slowly and carefully buiding to the middle of the map.
2) Then... wait for the boss to get into the left area.
3) And then... Build to the top of the map ASAP to lock and destroy the boss inside the left area.
After boss is gone - proceed as a regular level, since a lot of catapults will clear up half of the towers.

Hints:
1) Upon shooting a cannonball the tower will not shoot another one. Use this to your advantage and try to build as some cannonballs alredy hit the border and some are still flying to you.
2) After destroying the boss don't forget to pick power-ups to ease the rest of the level.