Well, since it's Kuritza you are up against, you can perhaps take comfort in the fact that you are being trashed by an excellent player. You might want to try a smaller map with other noobish players next time, if you prefer your lessons in dominions to be a little less brutal.
Soul Slay cares about MR, it's not completely random. Penetration items on the caster and MR of the intended victim matter a lot.