Story related and cannot be missed.
Simply survive through all of episode 9, “Carrier Full of Infinity”. There is no boss fight here, just wave after wave of minions and a couple of shooting mini games thrown in the mix as well. During the level you will constantly be forced to change teams to defend the ship. Just keep up the attack and use heroic attacks when you get in a bind. Always revive your partner if they are KO’ed too. This level might actually be easier to play in Co-op since the AI is pretty much useless on the shooting sections. The good news is that after each fight section you get a checkpoint so you won’t have to replay much.