I've been waiting for this. Those calculations involve a command squad (bonus FnP) and librarian (sanctuary to buff the invul save).

It also involves the primarchs being stupid. Horus should just use Worldbreaker to deny FnP, and Magnus will absolutely shred any version of Smashfucker in the psyker phase. The new Guilliman also has a chance to hit at D, which can end the fight.

Tag @Iratus as well. SM is a solid build, but he was leaning on his command squad and librarian for those fights.

ACtually with two cents of thinking Corax could winn any FFA between Primarch or anything else, how fight him when he doesn't show at all, so basically waiting for last pair fighting, whenat last one take down the other you quit the shadow and smash him hard, being untouch and unharmed you get the upper hand in the fight.