This is going to be slightly incomplete but helpful up to lvl 4 crystals.
To get crystals you need to be lvl 40.
You can speak to any of the following three NPCs.
Grand Magister Jurek in Giran - Magic Guild
Magister Gideon in Oren - Dark Elf Guild
Magister Winonin in Aden - Not sure haven't been there.
The mobs you need to hit so far I found are:
lvl 2 max Timak Soldiers, Timak Warriros
lvl 3 max Timak Overlords, Small fat Forest of Mirrors Ghosts.
lvl 4 max The two skinny Forest of Mirrors Ghosts and the Mirrors themselves.
If anyone can point me in the direction of the next levels I'd much appreciate it.