Regardless of how much damage it deals, the Soul Gem is just way too boring to be a 'Solo' trap. It needs more attack options, and some reactive defenses.
You missed (Level 5 / 2 round down = 2) + (Trained = 5) + (Stat = 4) + (Skill Focus = 5) = +16 giving you a 75% chance of success on each roll. I'm too lazy to do the math for the 6 rolls, but I'm sure it makes a huge difference.
