On low ranks and midranks to some degree warriors do rule the world, especially comparing to mages.

Actually, in most cases mage will just die to a warrior on low ranks without any chance to win.

What should you do as a mage:
Powerrank. The only exception to some degree is invoker. With all other mages powerrank. Conjurer - to 27 (angel/demon), shifter - to 3rd tier form, transmuter/necro - dont know, never played them

Seek your sleeks. You can start do this from 25, I think. With them life will be pretty easy and fun.

As assassin:
Go for hitroll and HPs do have enough time to kote/kan + dust them. If kote fails and you gushing - vanish away. Or dirt and then vanish.

General advice:
Post logs or describe certain situations. Will get concrete tips.

Read FAQs on Dios.

Make IC friends, who will help you. Gank with them. (yes, I know many will blame me for that, but if you can not be cool PKiller, try to gank).