Celtic Heroes

I know warriors are the best under these kinds of circumstances, but occasionally I will cast sharp shot or continue autoing when it says something like "Spearhorn casts cloak of fire" for example but than says it has been interrupted. Same situation later I do the same but still is able to cloak, I'm wondering if its luck or is there a way a ranger can interrupt a skill attack.

I interrupt when I'm using heroic gloves but is more successful when you use rapid shot. My rapid shot is upgraded just a bit but its fine without any upgrades and is still useful

It is possible for anyone to interrupt anyone else's skill attack- including mobs and bosses. The more hits, the stronger they are, the timing, all has to do with whether or not said skill will be interrupted.