Knights should attack on full attack to kill them before they can drain your mana. When facing more than three, try to reposition yourself so only two of them can attack you. These monsters drop enough loot that can be sold to make hunting them with 75/75+ skills worthwhile. Stealth Rings may be useful to kill them when there are three or more.
Paladins should keep distance and kill it like a Demon Skeleton, however keep an eye on your mana as you may die if it drops too low. Bringing some health potions is recommended.
Mages would have the worst time hunting, as they're immune to most elements and resistant to fire, if you do wish to hunt them it's a good idea to summon two demon skeletons, as they won't take much damage and let them do the work. You can add support them from the back using Explosion runes to speed the kill.
It is recommended that you use your mana as often as you can because it will be wasted by mana drain.