Don't you have Hitmontop though? I forget which version, but any version will be 2HKO'd IIRC. If you run Mach Punch however, I don't think the weakness will be all too bad. Just whenever you play against it, never switch as it 2HKO's anything which switches in, and it could be cool to Thunder Wave/Burn it with Chansey/Clefable. (If you run them, this is coming from memory.)
Mach Punch would be your main asset against Aggron though. Magmortar gets walled by either Rhyperior, or Altaria depending on the Hidden Power, but you don't run either. Just play around it! You can check the things which threaten your team quite well, so yeah. Its not like you need a counter for everything in the entire tier, its just impossible with only 6 pokemon.
On another note, I want to get into UU, but I need like... 36 UU pokemon EV'd. ;______;