Must piercing force is a must-get. It's in the lightning skill tree.
If I were you, I wouldn't get mind concentration.
Just a little rule I like to keep, to not get passive skills in anything I don't have skills in.. you'll just be taking up mastery slots. I mean, its fine for early game, but once you get the lvl 3x and above, leveling it 10 mastery levels just for leveling mind concentration will seem wasteful, when you could be spending the SP on something more useful.
Unless of course you plan on SP farming like a madman.
then by all means go for it.
The rest of your build is good, I've seen people with that combo (demarthl & moogie) do quite a bit of damage
