DPS hunters with fire-based pets (CDrag, or Lil Cruiser, or Baby Mech, or Mech, or whatever you have, goal is range and fire based dmg) stand at hint 5 and shoot East and West respectively. So have them firing at the Sides, but pets are blasting the North with those other fire-based towers.
Party:
1 Hybrid Monk (Tower Boost and Hero Boost stat as high as you can get them), standing on the build
2 Hunters with fire pets (auto firing, East and West), at hint 5
1 Summoner with cat/huntress pets (cat boosts the DPS and Huntress boosts the towers) at hint 2