I don't believe his ratios are worth building any AP whatsoever on him. His ult is .3 per hit, which is.. okay I guess? But it's not much better than a Heimer turret. And how often do you have multiple champs running from you in Dom? His E is like spirit fire damage, except point blank.. and without the DoT. Between E, Q and mallet slow it is really easy to lock down a single target consistently, no ult required.
His ult isn't the bulk of his damage anyway, his W is. More health and CDR is the way to go to capitalize on that. Plus, being a pure melee the tankiness doesn't hurt.
If I had to theorize an ideal generalist build I'd go something like CDR boots, Reverie (defense masteries to fill out the rest of your CDR), Sunfire, Odyn's, Mallet then Atma's to make use of all that bulk (and the free AS from Frenzy).