The runes I use are to increase the amount of damage I do since the items I get give enough crit. chance. and the alacrity runes help with the attack speed.

The runes for AP yi were chosen to increase the damage and further increase ability power.

All the items add up to do a lot of damage. Plus with the crit. chance over 100%, attack damage+speed+crit add up to a lot of damage in the end. I crit 700+ on enemy champions and attack 2 times every second. does not take long to kill an enemy champion.

All the ability power is to go to the heal and Alpha Strike, since every bit of the ability power adds to the damage it does. It is very successful. I've taken enemies to half health in one alpha strike, the gunblade and its lifesteal and spellvamp is to help keep you alive after the strike.

I use the build for most 5v5 games. I always come out top killer, usually even carrying the team. The only issue is going against anyone with a Thornmail, that will screw you over, because it ends up making you kill yourself. 30% damage return doesn't seem like much until you realize you have less than 2000 health, and 1400 damage every second, 30% of that is 420 damage coming back at you, and you have hardly any armor, so that all comes back at you.

AP yi is not a common thing to see. Its as surprising as AD Kassadin. But yet is still very deadly when Alpha striking into team battles, dealing loads of damage to all the enemies.