The new penance is to deal 50% of a monster’s health with brain burst and then get the killing blow afterwards. This is not difficult at all to achieve in a regular game without changing anything about the player’s behaviour or that of the rest of the party. I’d go so far as to say it will happen eventually for any psyker player even if they don’t specifically try for it. For instance if the psyker is the only player left and has to solo carry a boss encounter with >50% health left they achieve this by default just by using brain burst.
Now that the difficulty has been lowered to almost half there’s no reason to limit this to private games.