Looking for the title/author of an old sci fi book I read probably 30+ years ago.

This is all I remember: The protagonist is somehow transported to a different world where a kind of medieval class system existes. Combatants had advanced armor with plasma(?) swords and targeting in the helmets. There were very sophisticated suits and lower classes of these suits but the users didnt know even a fraction of the capabilities. The protagonist scavenged a lower class of suit from a fallen warrior ("He's wearing Villers old suit") and recruited 2 lower ranking warriors to fight in a coordinated attack. He swiftly rose in the ranks. There was also a wholly secondary plot that kinda sucked but I don't remember anything about that. I know it's not much to go on. Please do your thing.