Passive:

Full upgrades on agility, knowledge, might and critical shot

Active skills:

5/5 Aimed shot
5/5 noxious bolt
4/5 shadow veil, no upgrade on combusting decay
4/5 entangling trap, no upgrade on combustion tank
2/5 shadow piercer, upgrade shadow absorption only

Choose 4 out of the 5 skills above depending on maps you're going to be running. For example, when I run elite oltgar, I choose shadow veil, entangling trap, aimed shot of course and nox bolt. During boss fight you can switch traps to SP but given that oltgar mob is actually tougher than frostir, you'll be fine not switching trap with SP. Try experimenting with skills and see what works for you.