I think there can be a distinction made between Guerrilla Warfare (which is really a mode of carrying out war as a whole, a strategy), with Guerrilla Tactics, that can be used in phases of an otherwise conventional war. Certainly Sertorius used Guerrilla tactics, unconventional use of deception, ambush, small unit actions. I think he understood the Roman desire to force a decisive battle as soon as possible, and went to prevent it on anything besides his own terms. Because he was a master of maneuver and deception, he could often outfight his opponents even in situations where they thought they had the upper hand, by laying traps for them, stealing marches, etc. I think many of his Spanish forces definitely had a bigger guerrilla role, as their form of warfare was purely unorthodox, they did not want to fight, or plan to fight, in any way that a Roman would have found appealing, being fanatics for the war winning big battle.