Alright so I'm on my second playthrough as a Sith Sorcerer and I've noticed whilst playing PVP that I really cannot contend with a lot of players on here. Especially not the Sith Assassins who seem to have way more health than I do and their moves destroy 1/4th of my health in literally one attack. So my question is, what skill tree should I commit to on my Sorcerer? Also, what abilities should I focus on getting/using?

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You may be seeing the results of a gear gap. If you are below the "bolster" bar, then you are bolstered up to that level(1), but players who have stronger gear than that will not be "debolstered" to bring them down. And they may be more comfortable with their classes than you are with Sorc.

That's not to say that Sorc doesn't have problems in PvP. The class-to-class balance in PvP is a little off in places (just as it is in all MMORPGs).

(1) Bolster doesn't have any effect on "empty" gear slots, so make sure you have *something* in all fourteen slots(2).

(2) Seven pieces of armour, main weapon, off-hand weapon or doodad, two relics, two implants, and one earpiece makes fourteen slots in all.

Madness discipline is very competent for PvP, especially unranked where you can have a dotspread fiesta. The survivability is very good too, but if you are a very low level you may lack certain defensive abilities that give sorcs a boost in survivability.

That said, when facing melee, don't expect to be able to facetank them. That's not what casters do.