Kassadin Build

If they're stacking MR, replace something (likely a RoA or boots) with a void staff.
Aside from obvious, "It will take you an hour to max this build out" reasons, thoughts about it? It has good hybrid burst, enough mana that kass can use his ult as much as he wants, and quite a bit of tankiness.
I'd run hybrid pen reds, scaling AR yellows, scaling cdr quints, 3 scaling MR blues and 6 scaling cdr blues, and 21-9-0. This would give me a total of 30% cdr from runes and masteries, at lvl 18, so I can hit 40% with an elixer.
A max-stacked ult (which he could actually get, with this build) will deal ~600 magic damage. Then you can e, and get enough mana back to ult again
