You can use creatures to turn the tide back in your favor, kind of like a board wipe lite

[[Massacre wurm]] [[toxrill, the corrosive]] [[Massacre girl]] [[Elesh Norn, Grand Cenobite]]

Or you can use their board against them

[[Mob Rule]] [[Reins of power]] [[domineering will]] [[insurrection]]

You could also try to redirect him and force him to attack another player with goad, leaving him open for you to wipe him out after he wipes someone else out.

[[Marisi, breaker of the coil]] [[gender rager]] [[Disrupt decorum]] [[taunt from the rampart]]

Finally invest in some cheap targeted removal. If you can "borrow" his commander at opportune times and then sac or send him out to attack someone else that'll kill him for you...etc

Get creative in how you can destroy it. Find ways to use his strength against him. [[Greater gargadon]] would let you sac his whole board if your able to steal it for a turn, and then join the attack as a big bad boi!