Gem and enchant INT wherever possible.
Just aim for a socket bonus if it gives you at least 10 INT (or about 20 haste) for each non-red gem or when you have to activate your META.
Boss Fight:
SW:P -> DP -> VT (or in any other order) -> MF until ONE Orb -> MB -> reapply VT and DP -> DPS according to priority list ( VT > DP > SW
(below 25%) > MB with at least 1 Orb > MF > SW
(above 25%) (keep SW:P up by flaying)
Use Shadow Fiend on CD.
Use Dark Archangel on CD but only AFTER you've refreshed VT and DP doesn't need to be refreshed during the next 10+ seconds.
The reason you cast VT and DP once before you get ES is that you can be really unlucky and not get an Orb for 30+ seconds and your DPS will suffer EXTREMELY when you're just flaying with SW:P ticking.
PS: Mastery DOES increase DoT damage from Empowered Shadows. So many people seem to get this wrong at the moment. o_O