Arcane Comet is a relatively abusable keystone for Lissandra thanks to the very short cooldown and slow on [Q]. [Q] harass with Comet and Scorch will result in a lot of chip damage during lane.
WhyThe Ultimate Hat
The Ultimate Hat grants Lissandra fantastic safety and kill pressure. At 45% CDR and max Hat stacks, your ult is down to 32s.
Absolute Focus empowers your ability to harass in lane with long range [Q]s through the minion wave for even more empowered damage.
Scorch is a fantastic lane harass rune, and pairs incredibly well with Aery to keep the pressure up.
Biscuit Delivery is a great rune in match-ups where you're intending to sustain in lane for larger back timings. If timed well (when low health) in fights, the massive sustain can be the difference between life and death.
In difficult assassin match-ups, Perfect Timing is also a great rune choice for a cheaper Zhonya's.
Future's Market on Lissandra allows you to hit an earlier Rod of Ages purchase to begin stacking sooner than normal, making your mid game incredibly strong if you're not behind..