I discovered him after killing a dragon and going to learn my new Thu'um, he suddenly rose from the coffin and i attempted to fight him, he killed me in 3 shots, i tried magic the first, then melee (2-handed), then combined the 2 to. The combination kept me alive longer, but the results are still the same. Any ideas?
Krosis is one of the few masked dragon priests you'll meet outdoors. In fact, its not just outdoors but conveniently located on top of a mountain.
Thus, the easiest method of beating him would be to simply Fus Ro Dah him off the top of said mountain.
If you want to do it the 'normal' way, you'll have to bring ample fire resistance as he has a staff of fireball. He is also weak to fire so brining along a staff of fireball yourself or have ample fire magic would greatly help. Distracting him with summons (like the flame atronach) is a good idea as well.
A third way—the first two being knocking Krosis off the mountain or fighting them the normal way—is to get Krosis caught on the rocky terrain on the brink. Krosis usually won't attack while trying to navigate the treacherous terrain and if you can herd Krosis to a good spot, Krosis will stay in one place like a sitting duck.