well i signed up to the forums for the sole purpose of discussing Aggron seeing as the options people have mentioned appeared quite limited to me. Here is my take on mega aggron, has worked wonders for me as of late. This is designed to be used behind trick room support on double battles or single battles. just pick the second move appropiately.
*Trickroom Offensive / Defensive Pivot (COME GET SOME!)
Aggron @Aggronite
Sturdy
IV's: Spe 0
Brave 252hp 252 Attack OR Sassy 252 hp 252 SpD
-Heavy Slam / Ironhead
-Stone Edge / Rock Slide
-Superpower / Aqua Tail / OR *Roar/Dragon tail (*only to be used here alongside stealth rock)
-Roar/Protect OR Stealth rock
*For a non trickroom variant simply choose adjust the Spe IV's to 31 and choose the appropiate natures.
Analysis:
Aggron has acquired a variety of new assets that make him a top pick as a defensive pivot, while having the versatility of move-pool that is prevalent within the entire monster egg group. Aggron by default is a slow pokemon, and there is no reason to not use this to his advantage. Use him inside trickroom or as a trickroom counter by running a negative nature and 0 speed IV. The idea here is simple. Come in a known resisted move, and spam the mega-evolve button as soon as you get in. Sturdy is there to provide you some leeway if you are behind trickroom in the event you mess up your prediction. Even without trickroom Aggron is more than capable of soaking up damage and retaliating. 4x weakness are crippling, and using aggron as a lead might seem like the safe choice, but giving away your ace-in-the-hole mega evolution on the first turn is not typically advisable.
Once you get in your first priority should be to put up your stealth rocks immediately. If Stealth rocks are already in play you can go for the jugular with your wide variety move-pool, scout your opponent with protect, or aim for the predicted switch with roar or a switch of your own. A brave mega Aggron brings an impressive 414 unboosted attack power to the table all packed into a neat box of threat and utility, while a specially defensive variant provides a surprising amount of survivability in conjunction with filter to even some of the more powerful special moves in the game while standing beside a usable 344 base attack. To discuss some numbers, a filtered down non-stab flamethrower from any of the top tier dragon types does on average only slightly more damage than a resisted stab boosted draco meteor off the specially defensive variant. Here are some simulations from a hasty Rayquaza with 252 SAtk investment.
Draco Meteor 28.4 - 33.7% - 0.3% Chance to 3HKO
Flamethrower 39.5 - 47% - guaranteed 3HKO
Fire Blast 48.2 - 56.9% - 90.2% Chance to 2HKO
Dragon Pulse 18.6 - 22% - Possible 5HKO
These numbers are off one of the most powerful mixed attackers in the meta, all the while threatening with super effective rockslide which provides a guaranteed 2HKO.
Heavy Slam/Iron head is your bread and butter move. Once mega Evolved, it has been confirmed that Aggrons' weight increases from a somewhat impressive 360 kg (797 lb) to a usable 395 kg (870 lb), making it the 12th heaviest Pokemon in the game. But even though he has the weight behind his punches, the prevalence of heavier foes in the uber tier means that for the most part Iron head will be the safer choice. In fact, as a general rule, if the opponent weighs more than a Slaking 130.5 kg, Ironhead is the better of the two choices. Iron head also has a chance of flinching when used behind trick room support (something aggron could only dream of otherweise with his less than impressive speed).The second choice of move is for coverage, and the odd chance that you find an opening where you can use your rock typing which would typically only apply when facing a talon flame to abuse the 4x resistance to his priority flying type moves. In fact Aggron is one of the few true counters that Talonflame has to contend with; stone-edge for singles and rockslide for multi-battles. The third slot fills a variety slot for coverage and utility and prevents aggron from becoming total setup bait. Superpower provides him with a reliable counter against other steel types such as Excardill while aqua tail gives him the power and coverage he needs against the prevalent fire type counters in the tier without sacrificing defense and attack power. Superpower vs offensive 0 HP/0 DEF excardrill deals an outstanding 126.5 - 149.1% damage for an easy OHKO while only taking a meager 66.5 - 79.3%% from life-orb earthquake (an outstanding feat considering STAB and super effectiveness). Aquatail comes in close with 95 - 112.1% damage for a risky 70% chance to OHKO; making superpower the move of choice here unless provided with rain support; especially considering Excadrills' defensive variants. Stealth rock is not factored into these equations as the damage from stealthrock to Excadrill is negligible. On the other hand Roar is an option in this slot to be used alongside stealth rock for racking up passive damage by forcing switches. Aggron presence can almost immediately force a switch, and roar compliments this ability by not allowing the opponent to bring in a true counter, while forcing him to take additional passive damage because of his own switch. The last slot is truly dependant on the third moveslot, and while Aggron may typically spend his turns better attacking, stealth rock has become an uncommon move held by a select few, and as such it's mention is almost required. If you opt for stealth rock than you may opt for any of the moves in the third slot, but if you choose to run roar or protect then consider the coverage moves instead. The method to this madness is the tremendous loss of coverage, and the simple fact that combining a scouting and shuffling moves can provide a skilled player a chance to boost. If the opponent can predict the protect he might simply opt to boost instead and get in some bonus damage. You should typically only have 1 support move between protect and roar as they both fill a somewhat similar role; the former switching out possible threats before they ever get into the field of play, and the later scouting a good switch.
Counters (BALLS OF STEEL):
Mega aggrons greatest weakness is himself. If Aggron switches in on the wrong move he can potentially lose alot more life than he had first intended. Once Aggron succesfully mega evolves he becomes a completely different story. Mega-Aggron is truly the best if not one of the best stand alone pokemon in the meta. Aggron easily counters ho-oh taking less than 35% damage from stab boosted sacred fires all the while threatening with rock slide. In fact, aggron is so bulky that his specially defensive variant can only be OHKO by reshiram's signature move Blue Flare (125% damage) or fireblast (106%); the latter move just barely making it up to 106% all the while having to be careful of switching in to a dangerous stealth rock/rock slide combination which could severely dent him as he switches out to safety the next turn all for naught. But even this behemoth has his weaknesses.
Groudon has little trouble tanking Aggrons' sun-weakened aqua-tails, but still requires a 3HKO to defeat him, outside of the sun however, he risks trading blow for blow with Aggrons' aqua-tails taking a whopping 32-38% damage per hit. Aggron suffers greatly from burn syndrome, and the attacking variant quickly becomes a sacrificial pawn to soak up dangerous damage once successfully burned. Blaziken can come in on a revenge kill and threaten him with flare blitz provided aggron is at less than 50% hp, but risks trading dangerous blows otherwise as he only deals 41-50% (3HKO) damage per hit and will die a horrendous aqua-tail taking 71 to 91% damage should Aggron find an opening to attack. Hi Jump kick provides a more reliable 2HKO, but you must be wary of protect which is common with many variants which will ultimately spell your doom on the second attack should he have it. Even after a swords dance flare blitz does not secure the OHKO so play blaziken as a revenge only. Kyogre can come in on a predicted iron head and easily 2HKO with surf but takes considerable damage from superpower should he opt to simply attack and switch. Palkia can take every move except for superpower with stride on the switch, and threaten with a powerful fire blast for a 2HKO, dialga can do even better resisting both primary stabs but must watch out for super power or risk losing over half his life.
Team options:
(God mode, ALL OUT OF BUBBLEGUM)
Looking at Aggrons weaknesses your first thought might be to slap a Kyogre god of rain on your team and call it a day as puny fire elementals throw their unimpressive fireballs at your soaked God of steel. Well lets consider your strategy for a moment. From the calculations above we demonstrated how we could take a super effective fire attack and drop it into the 30%-35% (even lower for the physical attackers such as Blaziken) damage range by throwing rain into the mix. But have you considered the converse? When you do this those water attacks that were doing upwards of 40% now will be doing around 80% (some even much higher from the likes of the rain god himself). Not only that. Consider that filter reduces the damage of super effective attacks but NOT that of neutral attacks. Thus, adding rain to the mix, while severely hampering enemies fire type attacks will also conversely raise your weakness to water based attacks which can be just as common as the former.
(Team mode, WHY ALWAYS THE LADIES?)
Having said that, it is true that Pokemon is a team game, and trading evenly is indeed for nubs so if duking it out like a God vs all that stand before you is not your style, I would advise you to consider Rayquaza/Dragonite/Salamence or HO-oh who resist every single weakness that mega Aggron has for an offensive team OR Kyogre as a special wall to soak up those fire and water type moves on a defensive oriented team while providing you a screen of water vs the common fire attacks in the tier. Remember rain support also boosts the power of your aquatail. If you opt for Ho-Oh don't choose Kyogre for obvious reasons. I mentioned Ho-oh last because he is extremely weak to stealth rock and as such will require rapid spin support, alternatively Giratina may also be selected as a dedicated/support physical wall (depending on if you use offensive or defensive pivot) Because Groundon packs to much punch on the switch, and kyogre can't risk taking those hits.