Assassin/Ninja Help Lv40 Assassin

What kind of powder should I use on my armor and dagger
I have
14 fireI 14 fireII 2 fireIII
10 thunderI 11 thunderII 6 thunderIII
11 earthI 10 earthII 3 earthIII
10 waterI 12 waterII 1 waterIII
16 airI 11 airII 3 airIII

 

made a build for you
https://wynndata.tk/s/bzsw9r

but in general, at that level i would use either t3 or t4 powders. (t3 means tier 3 powder, t4 is tier 4 etc).
t4 are better though, and worth it if you put them on actually good weapons.
2x t4 gets you a powder special which you can use by sneaking and right clicking and you can charge it by melee attacking.
you need the powders to be the same element for the special however.

you could upgrade all your powders to t3+ and sell the ones you dont need if youre short on cash and then get more of the powders you actually want from trade market (i suggested 2x t4 thunder)

also don't powder your armor, the elemental defense it gives is very very minimal and the powder specials are also rather weak in comparison to the one on your weapon. after like lv 100 in an endgame build you could consider using powders on armor, but even then it's often just a waste of your money.