You encounter this boss in the main storyline and I'm pretty sure you have to beat him to continue, but I found a glitch that works perfect to beat this guy.
First, use a character that has a range attack such as Iceman, Deadpool, or Iron Man. I suggest using Ms. Marvel because here B power move does considerable damage to Titannus.
Second, how to do the glitch is you have to open the cells that are on the same wall as the entrance to the room, and you'll have to fight the experiments that come out. Second get into one of those cells and wait for Titannus to walk in(it helps if your character is the only one left alive). Next just leave and Titannus should be stuck inside the cell.
To finish off, just use your range attack and keep waiting for it to recharge until Titannus has been defeated. 15g