This unlock is a very confusing one. Imo it could be remade.
I, for instance, unlocked it by accident just as someone else already said here; was playing with an Arcane Blade, was ambushed, attacked someone (wasnt a skirmisher, although there was one on the nearby) and, BAM!; "you unlocked the skirmisher class!"... hmm ok, i guess.
I think i unlocked it using the shield talent that hits the enemy 3 times and blocks at the same time. Probably that skirmisher attacked me on the same turn, got blocked and than got hit by the arcane blade spell proc of arcane combat which almost always procs on that talent (it hits 3 times with shield, i think).
If that really happened than thats another way to unlock it (not an "easy" one though).
I don't think spells "consume" counterattack. What I think happened was the skirmisher enemy actually did the unlock for you. Aka, he blocked your attack, then counter attacked you from range.
Yeah, but the thing is; i never attacked that skirmisher directly. I attacked other enemy. The only thing that hit that skirmisher was the spell proc from arcane combat.
I dont know exactly how, but i'm quite sure the spell proc from arcane combat behaves a little differently from any other common spell that you cast. Maybe the spell do count as a consume hit because it was generated from a physical hit proc.
Or maybe is just a bug. Anyhow, its a really difficult situation to test it out, you have to use that shield talent on someone else, the spell proc has to hit the skirmisher that is behind that someone else on the same turn that he attacks you.