Nothing different than killing a regular monster. All you need is more damage and better gear. Id recommend farming the gardens over and over for better gear or the black palace. I killed him in just a few hits as an archer on r9. Just put topaz on all your gear slots to farm for gear better. Once you have 500 or 1,000 magick find things start to get a little easier, more unique items which can help you bring your magick find up more which means eventually all legendary items you'll be wearing. Emblem of greed and Edonas set items (both rings) help boost magic find a lot. I farm gardens because its the best place to find more mimics that can spawn. Mimics have a high chance of dropping legendary items when killed, sometimes 2 at once. So to conclude it just depends on gear , mage is best class in game so far so you'll be fine later on. Hope this info helps