I thought a Primarch vs a Chaos powered space marine was a comparable or possibly even more fair match up than The Emperor vs a Chaos powered Primarch.
Nope every instance of primarch vs marine has been a rape stomp
The only exceptions have been lion vs luther (and that may be sorcery that stopped the lion) and kor pheron (could be wrong) magicaly sucker punched guilliman and had him frozen but still lost
Few physical encounters go well
Although One of the elite spacewolves is asked to try something with the lion in guillumans presence....swings at him but lion catches blade with ease, guilliman doesnt know if hed be able to do the same
Angron sits and talks with a dying loyalist world eater he kills , comments the man managed to scratch his neck and respects him for it
But overall any marine vs primarch has been easy stomp...shit they kill them em masse!
Russ sent elite packs of space wolves to watch over other primarchs and in general they were slaughtered vs disloyal primarchs with minimal fuss
Gulliman himself is ambushed in the nip by a group of fully armed elite alpha marines in close quarters and still kills em all
Corax rips through the demon enhanced world bearers so quick that logar proceed to get his ass whupped to stop the slaughter
Overall the human food chain goes
Big e >primarchs>custodes> marines
Even chaos enhanement wouldnt alter that much. Abandons only hope is suprise and the power of the 2 uber weapons he wields