Powerword: Flames deals periodic damage each round, and is an AoE.
Powerword: Kill deals direct damage based on your opponent's current HP, so its damage declines over the course of combat but can do up to 600k vs mobiles.
Powerword: Stun lags opponent significantly but also has the longest cooldown and is thus saved for the end of combat.
Soulsuck whenever powerword is down.
For classes that take longer to kill pw flames is usefull. Pw flames is an area attack, so be careful when using it. If you are going to use pw flames try to be the oppisite alignment of your oppenent and make sure you have a jade gem forged on your equipment.
Nitemare replies to you 'then pw kill once and soulsuck then stun'.
Nitemare replies to you 'against taross i suppose if they are magic I'd use blind, but none ever are, so I never use blind (Ed note: blind doesn't stop their midround anyways)