What counts for Expose Weakness?

Id assume any damage. Pretty much if you go into combat with that enemy, you proc it...... but that is just my assumpion, hard to tell.

Yeah, I'd love to get an official response on this.

No one knows?

I take it on junglers.

You go in, deal damage, and then your laner(s) deal extra damage.

Given that Sunfire cape counts for being in Combat for Grasp of the Undying, I suppose it counts for Expose Weakness too?

Did you check the Wikia?

The wikia says abilities.

Case closed.