Mecha-Bruce is an enhanced version of Bruce. The gimmick of the fight is that Mecha-Bruce becomes increasingly Better, Stronger, Faster. He does this every other Chomp.
His Chomp, Chomp, Chomp is also upgraded in the sense that Mecha-Bruce will stun your character in place. You cannot escape the stun, but you can use Immunity-granting spells and abilities.
The mob is essentially a DPS check à la Patchwerk in Naxxramas. Few classes/specs below ilvl 510 will be able to successfully defeat him.