Currently, Malleus Monstronum is a badly designed penance because there is no exciting way to do it other than finding a pre-made group that agrees to sit around and protect you while you Brainburst a monstrosity for a few minutes all by yourself.
For a team-based cooperative game it is pretty bad design, but it didn’t have to be! As I kept thinking how to actually make this Penance work in this game it hit me: there’s a Feat that debuffs the enemy after a Brainburst for a few seconds. So what if friendly attacks during this period actually counted for Psyker damage? Then your team could engage the Monstrosity, too!
Wouldn’t that be fun? If your whole team could share equally in this activity? In this team game?!
If you are a crafty Psyker and are able to manage your Peril accordingly, I imagine this Penance could actually be won in pugs, too. As long as the Psyker attacks the Monstrosity first and then keeps up the debuff.
If I had my way, then Brainburst would apply the debuff as it charges up (I imagine the debuff would charge from 0% to 25% extra damage parallel to the BB itself) already, with the actual Feat only extending that full debuff period for a few seconds after each Brainburst. But that’s just my 2 cents.