Tier 1
Power hungry - You almost constantly getting health globes and 10% AP damage is huge coupled with 100% mana generation for 20 seconds.

Tier 2
Dominance - This is your defensive talent. This allows you to play agressive and go for the kills, knowing that you are healed ready to take on the next Hero.

Tier 3
Seeker - This is required talent and adds extra burst to your bread and butter. Also adds the damage to the consective missiles from Mirrorball (Tier 6 Talent).

Tier 4
Disintegrate - Used in the kill rotation, low cooldown and clears creeps. What a range as well.

Tier 5
Glass Cannon - Lose 15% health for a massive 15% Ap is huge. added to your power hungry buff thats 25% more Damage.

Tier 6
Mirror ball - benefits from seeker and essentially adds massive burst and a wide clear if needed.

Tier 7
Tal Rasha's Elements - Essentially 20% AP giving you 45% AP damage if cast correctly for each move used.

Rotation
MAgic Missiles, Orb, Missiles, Disentegrate

Always keep a mental note of the abilities cast to maximised damage.

Note
Range of missiles is huge. Sniper the towers with mass damage and never get hit by them.