I've never managed to build on top of the bridge and have the runs be successful, so here is the build that I do when farming Spear of Hearts and Heart Fragments.

I do use a Warden equiped with a Huntress Guardian that comes in once upgrading is completed. My Huntress Guardian is nothing special. Basically 400 or more in each builder stat. Warden stands next to DSTs and Wisp Dens under the bridge because these are the six towers I want boosted most. 

Arrows
Blue = Cloud Tower direction
Black = 4 DSTs direction

First Wave
Get mana and unlock secret for additional mana
Build Auras, Buff Beam, DSTs, Wisp Den, Cloud Towers
Buff beam goes as close to under the bridge as possible, towers go on buff beam, obviously. :)

During 1st Wave
Build 6 Shroom Pits

Remaining Waves
Tower Boost and Upgrade. After upgrades bring out Summoner with Huntress and Huntress Guardian if you want waves to be faster or do I like I do and bring out my Builder Warden with Hunt Guard.

Boss Wave
Bring out Builder Warden, DPS Monk with Eye pet, DPS Monk with Eye pet and hide under the bridge. During Boss fight, Hero Boost and Tower Boost and wait for bosses to die.

Video
Maybe...