Aw, poor marine.
Ummmm...I'm calling bullshit on that one. MC is powerful but not as strong as any marine, espically not an Angry Marine.
I'm calling plausible. A human with a power sword is a human with a power sword. Considering how MC is on a basic level an incredibly well trained human with plenty of combat experience, it is entirely within his capabilities of taking down a space marine when armed with a plasma sword. Also considering the space marine was armed with a power fist and chainsword (no bolter!), in that particular fight MC would have been faster and be able to get close - within the marine's guard and promptly stab his face.
This. In 'Alpharius' (IIRC) an enhanced human soldier from a regiment known as the Lucifer Blacks fights the Primarch Alpharius in one on one combat with a NORMAL sword versus a POWER sword (A PRIMARCH's power sword, mind you, so probably artifact quality weapon here) and fought him to a standstill. The fight ended because the Lucifer Black stabbed Alpharius through the side seam of his breastplate, piercing (one of) his hearts. Alpharius then laughed at him, broke the sword off, and cut the man in half.
Primarchs are often touted as 'To a Space Marine what a Space Marine is to a man'. I imagine that, if a somewhat enhanced human (like MC would be) can fight a Primarch and only lose because he didn't have a power sword, then an enhanced human with a plasma sword could probably kill the fuck out of a marine. Just depends on whether or not the marine had a ranged weapon and forewarning.