You can tell its age by the length of its iron horns. Hippowdon can effortlessly switch in, wall it, and beat Mega Aggron with repeated uses of Earthquake. The horns on its forehead are now shorter, and it has a third, wider horn on its nose.
Superpower has similar coverage to Earthquake, and is particularly useful against Mega Kanghaskan. Dragon Tail and Roar can be used to shuffle threats; both moves have their advantages (dealing damage vs forcing out Substitute users) but they both come at the cost of a precious moveslot. Although you've implied this in your calc, bulky Kyogre is an issue as well since it naturally outruns RPggron without a boost (unlike Groudon), and Head Smash will fail to OHKO.
If Heavy Slam's inability to cover certain threats bothers you, Iron Head can be used. Togekiss similarly functions as a partner to Mega Aggron, and with the added benefit of being immune to Ground-type and the ability to also spread paralysis.
Omega Ruby
Aggron claims an entire mountain as its own territory.
SoulSilver
Generation V
Unova B2 W2 : #168
Black
While seeking iron for food, it digs tunnels by breaking through bedrock with its steel horns.
Its forehead plating extends past its upper jaw in a small point. As mentioned in Pokémon Sleep, Aggron has been observed eating iron ore on rare occasions while it sleeps.
Brilliant Diamond
While seeking iron for food, it digs tunnels by breaking through bedrock with its steel horns. They are used to determine the Aggron's age. As a result, it greatly appreciates Wish support to heal off damage. Its body contains a high concentration of iron.
252 HP / 252 SpD Mega Aggron: 181-214 (52.61 - 62.2%) -- guaranteed 2HKO</li> </ul>
<p>As you can see, it's pretty much impossible for physical attackers to OHKO Mega Aggron without a boosting move, and even Special Attackers struggle to take it down in one shot.
FireRed
It claims a large mountain as its sole territory.
The more wounds it has, the more it has battled, so don't take it lightly.
Sword
Aggron has a horn sharp enough to perforate thick iron sheets. It mercilessly beats up anything that violates its environment.