You are the Demon Lord, doing what you do best, governing Royal Makai and fucking your beloved husband. The royal shagging however was cut short when you heard one of your subjects burst into your royal chamber clearly having run out of breath. "*HUFF* *PUFF* MY MISTRESS" The Baphomet barely says "*huff* *puff* what's wrong?" you ask also being a bit out of breath from loving making you were having with your husband. "WE'RE UNDER ATTACK" she says with a fearful look in her eyes "*sigh* the order again? they always try and they always failed when will they get that and leave Royal Makai alone" you say clearly tired of the order's crap. "NO IT"S NOT THE ORDER. I DON"T WHO THEY ARE BUT THEY ARE SHREDDING THROUGH OUR DEFENSES" the Baphomet responds shaking in fear.
With that you put your clothes back on and rush to one of the castle's balconys to see a terrifying sight. You watched as some of your best soldiers were beaten by some 8 ft goliaths of what you assume to be men. They were all wearing some sort of armor that you haven't seen before as well as holding strange metal devices that seem to fire giant projectiles that were about the size of a sword hilt. Some of the warriors were black with skulls on painted their helmets, blue without their helmets and some were even in expensive looking golden armor.
However the one that caught your attention the most wasa soldier that was coloured red and was on top of a hill. You watched as he kicked a minotaur off of it with little to no effort and shouts "DIE SCUM, PURGE THE HERETICS, TAKE NO PRISONERS, FOR THE EMPEROR!!!!"
(If it wasn't obvious enough, this WWYD? is supposed to be a Warhammer 40K crossover)
As funny as that is Beyondrapture I don't think Space Marines are actually overpowered in their own universe they are quite often get their ass kicked in by almost all the major enemies such as the tyranids Orcs And khaos( Tau are space communists, Eldar just dickish space elves and Dark Eldar just super dickish BDSM space elves). Unless you're an ultra Smurf then everything is always totally awesome and you even get your primarch back. Not to undercut the Imperial Guard, they serve a very important function in the Imperium of man. At least it wasn't primaris Marines those are the super superest super soldiers those definitely have super-mega adamantine Infinity plot armor. However this is all irrelevant because the most Superior faction is by far the adeptus Sororities, so long as you don't call them bolter bitches because you will end up in a promethium bath.