Just remove the damage all together, don't reduce the ap scale, don't nerf his other spells just.. remove... the ult... damage. That way he'll become a true assassin, crazy movement speed and gap closers, and then when he's assassinated his targer. Poof! he ults out, that's how he should of been designed like, his ult was meant to be an escape, a redo, when you fu'cked up you can get out no harm done. Yet riot made it do ap damage, and now people use it to engage rather than escape.
This change imo will make him a fair champ with counter play.