Permanent invisibility, as long as you do nothing. Hitting reveals you for 8 seconds, being hit for 5 seconds.
The Phantom does not win fights, it chooses when they start. You cross the arena unseen, you take your position, and you open on the weakest target — after which you are an ordinary PvP kit for eight seconds.
Careful: invisibility hides neither your particles nor the items you hold. Attentive players still spot you.