Mega Aggron is tied with Shuckle for highest base Defense at 230, tied with Shuckle. Unlike Shuckle, however, it can actually hurt things with its s more than triple its HP (Aggron, 70, Shuckle, 20). Being pure Steel makes a great defensive typing that resists 10 different types. The problem with that is it is still hurt by some of the most common attack types, Fire, Fighting, and Ground. How are you going to deal with those hits? MegaGron gets a new ability, Filter, which is basically Solid Rock in that it takes only 3/4 the damage from super-effective attacks. This is like the one Pokemon who can tank a super-effective High Jump Kick from Mega Medicham and then proceed to kill it with Heavy Slam.
252 Atk Pure Power Mega Medicham High Jump Kick vs. 252 HP / 0 Def Filter Mega Aggron: 253-298 (73.5 - 86.6%) -- guaranteed 2HKO
252+ Atk Mega Aggron Heavy Slam (120 BP) vs. 0 HP / 0 Def Mega Medicham: 261-307 (100 - 117.6%) -- guaranteed OHKO
The only thing I would watch out for is the fact that its Special Defense is pretty much just average and there are other Mega Pokemon who can hit other stuff hard like MegaZard or MegaCham and aren't completely tied down by a base 50 speed. However, anything that can tank a MegaCham HJK is a behemoth and will wall everything. It only has two ways of recovery and that is through Rest and Wish. Rest can be combined with Sleep Talk, but its unreliable, or with Heal Bell support. Wish can be passed to it by a Pokemon like Florges who can tank Fighting attacks for it and is a good Specially Defensive combo.
You can check out some more sets here.
One set I like in particular is this one:
Aggron @ Aggronite
Ability: Sturdy / Filter
EVs: 252 Def / 252 Atk / 4 HP
Nature: Adamant / Impish
- Iron Head / Heavy Slam
- Stealth Rock / Thunder Wave / Fire Punch / Stone Edge / Ice Punch
- Dragon Tail / Thunder Wave / Roar / Fire Punch / Stone Edge / Ice Punch
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