Yeah I've used Mega Aggron a lot recently too. I support it moving up to A- for sure, it's actually great as it can set up SR against the most popular defoggers Crobat and Mega Aerodactyl. You can also easily kill Empoleon and I ran Toxic which beats Defog Gligar, so it really is one of the most reliable SR users in the tier. And it does counter some very significant threats which dynamic listed above. However I don't think it's quite A material because of the bulky water problem (Toxic is really great but a Scald burn makes it useless). Also it doesn't really get to use its incredible physical bulk to its fullest because it's weak to the three most common physical attacks: Fire, Fighting, and Ground. So yeah A- is great for Mega Aggron but A is too far for now.
Maybe A will be more appropriate in the ORAS metagame because it counters a lot of the new megas but who knows.
Maybe A will be more appropriate in the ORAS metagame because it counters a lot of the new megas but who knows.