Please follow YouTube video for visual representation of breakable parts.
Boss fights are at time stamps below.

#0 @1:50
#1 @5:58
#2 @10:32
#3 @15:38
#4 @18:48
#5 @24:04

Sikkim (Rocket punch doll) seems to be the best choice as he has high damage output, and precise aim. You can easily target each part even if it is behind other parts. All respawning parts only need to be broken once. First and last boss can be killed normally as all parts break automatically throughout fight.

@1:50
- Phase 1: Left arm
- Phase 3: Right arm
- Phase 4: Head

Boss's parts will break automatically even if you only target his head. Just continue to kill him as normal.

@5:58

- Phase 1: 4x turrets (only need to kill each once. Don't worry if they respawn)
- Phase 2: 8x rotating parts
- Phase 3: Kill boss as normal. 2 parts will break off automatically
- Phase 4: 8x rotating blades (only need to destroy each one once)

@10:32

- Phase 1: 3x turrets, 2x shields. For center target make sure you don't do too much damage to the boss itself otherwise he'll go to next phase and the turret won't break.
- Phase 2: Back left turret, back right turret. 2x front horns. Destroy the turrets first, then either left or right horn. Once you destroy one horn the other will break automatically.
- Phase 3: 3x turrets. Destroy 2 outside turrets first, then destroy center one.
- Phase 4: Right shell, left shell, head. Destroy in any order
- Phase 4.5: 2x cannon pods, 1x engine. Destroy cannons first, then destroy engine.
- Phase 5: Kill Boss as normal

@15:38

- Phase 1: 2x dump cores, 4x Tessa chains, 1x magnet. Destroy magnet and dump cores before destroying chains. May need to sacrifice a few dolls to destroy it. Must kill chains before dealing too much damage to boss otherwise they will disappear.
- Phase 2: 2x dump cores. Destroy cores, then destroy boss.

@18:48

- Phase 1: 6x spotlights, 2x front attachments, 4x engines. Destroy spotlights first, then the other parts.
- Phase 2: Destroy boss as normal

@24:04

- Kill boss as normal

Congrats on your new achievement!