Caedus did fight that battle in his personal torture chamber with more than one trap. Luke's surprising Caedus is pretty much irrelevant as Ben, (the fool) told Caedus before luke ever went on the attack, that "Dad is standing right behind you" I think that is as much fair warning as any combatant ever got in any fight. Just because Caedus didn't turn around, i don't think we should count surprise as an advantage to luke.
Lastly, because Luke didn't debilitate Caedus, he merely caused him a PAINFUL wound, it could be argued that the opening salvo actually gave Caedus an advantage, as he fights better when hurting.
Okay, this time is lastly: Caedus reflects on the fight he had with luke later, and considered himself outclassed.
Had a thought in this thread though: With the propensity that luke has for getting injured in every duel (to make him look human) he might now actually clear this gauntlet. Take enough chunks out of a guy, and what do you have left?