what champs are good at tanking or countering ap damage?
if teemo can counter ad from autos what's his ap counterpart?
inb4 downvotes for no reason
if teemo can counter ad from autos what's his ap counterpart?
inb4 downvotes for no reason
anything who buys a single MR item?
I mean Galio, but really just adaptive helm is really op against poison and sustained magic damage.

Tanks are designed to counter AP burst damage.
Build MR.
Teemo is largely a cheese pick designed to tilt the enemy top laner. As long as you pick something that scales into late game and hold on until then, you'll be fine for team fights and his team will be screwed
basically.

galio and mundo are specifically anti ap champions.
Well, AP is a stat, not a damage type, but silences and spellshields prevent damage from abilities, which generally scale on AP. If the ability uses a projectile, then intercepts will work against it, but those work on projectile-based autoattacks as well. Many abilities are also dodgeable, unlike autoattacks, so mobility can make a big difference.