This combination of health, ability power, magic resist, and magic penetration will allow you to crush foes mid and boost survivability throughout the game.

The suggested 21/0/9 masteries for Lissandra seem fine.

Get your Rod of Ages as quick as possible. Maintain a balance of carry and tanky. Main build adds Liandry's Torment for some magic penetration and DoT, then a Rylai's for health, AP, and its slow to supplement your ult and Liandry's. WotA adds survivability and makes your ult awesome. Grab more penetration and AP to finish. If else, check out more tanky and more carry.

Grab all 3 first then obviously max Q and R when you can. Get W next and finish E last, as it is only your get-away.