It is possible for the Incubus to wear any armor, though they prefer leather armor, they wouldn't be restricted to it. An Incubus can gain tremendous power after training. This does take more than millenia to achieve. There is nothing that says an Incubus can't "Upgrade" himself from lesser demon to greater demon by training. Incubi reaching this level would be considered not normal Incubi. It's a little bit in the area of equivalence with the Queen of Torment/Suffering, etc. Also, Incubi having reached this level, would have no trouble charming mortals. Un-protected ofcourse. An Incubus at this level would also be able to control tremendous amounts of persons at the same time. And it would consider it able to charm powerful things if it was wore down enough.
Incubi are AT HOME during War, due to their ability to feed on Pain, Suffering and Death of others. They would be More powerful there than anywhere else, as i mentioned before.
And, due to the lack of knowledge of the Lifespans of Demonic Entities, Incubi could live LONGER than Eredar. Due to the massive amount of Fel Energy in them from Sargeras, they would easily be able to feed off of other's lifeforces to fuel both their own, and their powers, giving them a short-term boost in strength, as well as an increase in life span. Though this would begin to force them to HAVE to feed off of other beings, leaving bodies in their wake, all wasted and wilted from extraction of their energy and life force. So on and so on.
Eredar are not skilled in melee combat, not as skilled as an Incubi would be. So logically, if the Incubus was able to get into close combat with the Eredar, the fight would undoubtedly quickly switch pace from the Eredar's lead to the Incubus'. It wouldn't take that long for him/her/it to wear down and kill the Eredar.