Which mystic do I rank up

SlayerKing

I don’t have any decent mystics besides sym supreme and I wanna rank up either guillotine or dormammu

MarkWard

Sym supreme is very useful champ, and among guillotine & dormammu, the second is better.
The main thing in dormammu is his energy control from one hand, and from the other one - his damage is pretty low;
Guilotine might be useful versus champs with regeneration, she may reverse heal ability into damage and increase damage with a help of souls (don't exactly remember whether she needs to be duped or not) .

So, I'd reccomend to rank up dormammu (after sym supreme surely)