Defeat the indicated Boss as an Autobot (to unlock an Autobot Boss) or a Decepticon (to unlock a Decepticon Boss) in the Arena to unlock them as a playable character.

Character	How to defeat
Air Raid ( Autobot )	Defeat him in Streets stage #6 (Arena stage #18).
Barricade ( Decepticon )	Defeat him in Stadium stage #6 (Arena stage #6).
Breakdown ( Decepticon )	Defeat him in Dam stage #6 (Arena stage #24).
Cyclonus ( Decepticon )	Defeat him in Streets stage #6 (Arena stage #18).
Grimlock ( Autobot )	Defeat him in Purification stage #6 (Arena stage #30).
Hot Rod ( Autobot )	Defeat him in Dam stage #6 (Arena stage #24).
Jazz ( Autobot )	Defeat him in Stadium stage #6 (Arena stage #6).
Motormaster ( Decepticon )	Defeat him in Sewers stage #6 (Arena stage #12).
Shockwave ( Decepticon )	Defeat him in Purification stage #6 (Arena stage #30).
Ultra Magnus ( Autobot )	Defeat him in Sewers stage #6 (Arena stage #12).