This boss fight could be really tough since cheats are non-existent and we are unable to use the 'free cheats' trick any longer. The easiest way to defeat Malteus is using Church (Katana 'emo' chick) with her Bloodbind spell (RB) . Stay behind the 'sarcophagus' sort of thing while using Bloodbind most of the time. This keeps:
*Those phanto-fuckers at bay instead of flying around.
*Maltheus can not pass the sarcophagus.
Cover so you don't get hit by shrapnell from his shells, use Bloodbind, and melee the fuck out of Malteus. Repeat this method some times, revive some team members and return to the sarcophagus. Rinse and repeat until he starts yelling how he loves his children. Malteus killed, end of story.