how do you counter ap ammu?

Don't let his Q hit you and he's entirely useless, like a really bad assassin. Banshee's Veil blocks his Q. Merc Scimitar and Mikael's cleanse his ult. Zhonya's can help if he bounces in on a nearby teammate.

Most of his damage is percentage based, so Magic Resistance will help you more than health. Also, he'll be pretty squishy, so killing him fast will also help.

MR. Lel. but srsly, just dodge his q. if he cant get close to you he has nothing.

build mr early, and build items like to disengage from his cc also hlp along with

MR? Dodging his grab?

If its jungle APmumu, then just counterjungle his sad backside out of his jungle. He is squishy earlygame and only starts ramping up the damage midlane onwards. Just by invading him and stealing his blue you utterly ruin him.