I was 895 (gloves & sephuz because that's all I got) at the time of the kill and while the elemental can really save you if the runes spawn in the middle of the mobpack, I felt as if I was lacking the overall dmg.
The strategy that did it for me was:
Use purgatory phases to pump as much dmg into raest as possible (hero, pots, everything), usually you can cast in peace here.
Kite karam in a circle around raest so you don't have to worry about slowing him in the last phase since the voidzones will to that for you -> you can focus all your dmg into raest
Use Lonely winter for more dmg and pray for not shitty rune spawn locations