Your first major challenge is Rocksteady! He's a tough opponent but has predictable patterns. Rocksteady primarily uses a charge attack and a ground pound. When he charges, dodge out of the way and then counter-attack as he recovers. His ground pound creates a small shockwave; jump over it to avoid damage. Focus on landing quick combos and then dodging away. Don't get greedy with your attacks, as he can punish you severely. If you have a full special meter, now is a good time to use a Super Attack for significant damage. In co-op, have one player distract him while others attack from behind. After defeating Rocksteady, you'll receive a large pizza for health and progress to the next area.
