I shouldn't rank lightning and mana shield? O_o
Well, I was never sure to rank lightning (a friend just told me to) but doesn't mana shield's efficiency go up with ranks? Right now, a 10 hp-hit on me is like 30 mp-hit on my mana shield.. :/
It does increase but unless you have a high amount of magic attack or the reforge to supplement the mana shield skill, it falls short of just using Hp and your defense to tank. Basically you'll be consuming more potions under the influence of mana shield than without (and thus losing more time/money in the long run).