Have you tried Poison Armour and Status Weapons? Most of them avoid at maximum taking any hit, be it magical or physical, your best way is to use a bleeding weapon (Say, a weapon with 4 savage babe), and use status inflicting skills, like Envenom, Hammer Fall, Bash, Double Strafe, Melody Strike and many others. You also must have most of your equips on BM to keep changing between shields, because if you mitigate way too much Sonic Blow damage and hit them, they will avoid being close and start abusing SBK, which you will need to get a horn card to lower it.
OR, you can just get Valkyrie Randgris Cards/Professor and make him dispel the Assassin, as you said, they have countless buffs, a simple dispel can bring all their work down in a matter of seconds, then just assault him with magical spells till they die, but I don't think it will be possible, since he will disconnect to rebuff at the same time you debuff him.
Since you're a hunter, you should start abusing traps on PvP, I'm pretty sure 100% Assassins fall on traps(Even if they know it) and get revealed, don't use Ankle Snare, since you do not know if he activate it until it fades and they still can SBK you, you can revolve from: Shockwave Trap (Mid fight to outlast his invisible), Mine Trap ( I believe it deals stun), Claymore (The utility of this trap is that sometimes people click on it believing they are targeting you, and accidentally hit the trap, showing their positions). If they aren't falling for them, just Arrow Shower around the trap, it moves it. When you're arrow showering, there is a GIANT chance that he will appear, so keep changing between effect Bows and Arrows. Prefer Silence Arrow and Stun Weapon, if you silence, he won't be able to cloak/uncloak at his will, which causes him to waste a good amount of SP, and if silenced, he can't one hit you. Stun if for obvious Reasons. Since I know most of the Hunters are Falconers, I'd say every time you stun him to cast at maximum falcon assault, most of the times that end up with 60% of their HP.
Well, this is in theory, in application it will be a lot harder.