[Agema]

Hm, yeah, should have made clear.

Your average Tart probably has items to give it MR over 20, if not nearly 30. Your chance of beating its MR with an unboosted spell or mind blast will be well under 5%. Add penetration boosters (Eye of the void, rune smasher, stellar focus etc.), and if in battle use communion to get additional astral level. You can easily get +5 penetration or better, which will double your chances if not better. After that, it's simply a case of firing so many that eventually one works.

Also try mind control instead of soul slay. Then you'll have your own Tartarian.