Yeah, that works perfectly. Beat the monster down until its almost dead, use your custom spell, and get the soul.
incase it resists shock damage, i use a weakness against fire damage 100% and shock damage 100% and frost damage 100% and magic 100% for 10 seconds, and immediately after, i cast my soul trap shock spell.
and sometime, i cast the weakness spell twice on him so its weakness 200%
i has the "devils spell" which cast all types of damage 6 points for 6 seconds.
It killed guards in one hit, before I leveled up to level 25 in some kind of leveling growth spurt.
im still not done leveling, and because i chose apprentice sign, any necromancer and the mythic dawn can insta-kill me.