He has a very balanced 50%ish winrate atm. He's not worth nerfing. Remember he gets silenced when he spams too much and does little to no damage when its empty. Attack him at those points but never when he's near half Heat. Use MR Runes, SHIT on him in level 1-3 since he's super weak and vulnerable without 3 abilities (he needs to spam all 3 back to back to raise his heat). It might be best to pick a ranged character like Gnar, Quinn and especially Ryze since Rumble has no gapcloser. Someone like Panth cannot escape easily so I think you're just counterpicking yourself personally.