Altaria is seen as the weak little sister to Sylveon and Gardevoir. It doesn't hit nearly as fast and lacks the special bulk of the two others. Plus, the Dragon typing is a double-edged sword — while it gives Altaria nifty resistances, it adds a dreadful Fairy and Ice weakness. However, it has its perks as a bulky Tailwind setter (which Togekiss also does) and has unparalleled coverage for a Fairy-type. Here are Altaria's weaknesses and major threats that will be targeting it:
Ice – 
etc.
Poison – 
Steel – 
Fairy – 
Therefore, you want something with Psychic coverage for Conkeldurr, Nidoqueen and Venusaur, Grass or Electric coverage for the bulky Waters, Fire coverage for Escavalier, Scizor, Ferrothorn, Mawile and the other Steel-types, Steel coverage for the Fairies, and Ice or Rock coverage for Thundurus, who is a particular danger with Thunder Wave. Here are some 'mons that can work well with Mega Altaria:
Aegislash@ Life Orb
Modest | Stance Change
180 HP / 252 SpA / 76 Spe
- Flash Cannon
- Shadow Ball
- King's Shield
- Wide Guard
Few Pokémon make better partners for a Tailwind Altaria than Aegislash. This is the standard TW Aegislash build, outspeeding base 130s in Tailwind. Life Orb makes Aegis hit like a train and OHKO some Sylveon expecting the Weakness Policy or Colburr Berry variants. Flash Cannon targets down those Fairies and Wide Guard stops Hyper Voice. Be warned, however, that this lead is quite weak to Bisharp.
Heatran@ Safety Goggles
Timid | Flash Fire
4 HP / 252 SpA / 252 Spe
- Heat Wave
- Earth Power
- Flash Cannon
- Protect
Another offensive powerhouse under Tailwind, Safety Goggles Heatran is a fabulous partner for Altaria, who is quite weak to status. He checks Spore spammers with Heat Wave, Steel types with Earth Power and Heat Wave, and Fairies with Flash Cannon.
Volcarona@ Lum Berry
Modest | Flame Body
252 HP / 236 Def / 12 SpA / 4 SpD / 4 Spe
- Rage Powder
- Tailwind
- Fiery Dance
- Bug Buzz
Volcarona is a cool option if you don't use Altaria as your Tailwind setter. This spread survives Salamence's Hyper Voice, Landorus' Rock Slide and Politoed's Rain-boosted Scald. Rage Powder keeps Altaria safe, while Fiery Dance and Bug Buzz are dual STABs.
Breloom@ Focus Sash
Jolly | Technician
4 HP / 252 Atk / 252 Spe
- Mach Punch
- Bullet Seed
- Spore
- Protect
Here he is, the King of Tailwind himself. Fastest Spore in the game, right here. He also does an amazing job at checking Steel-types and bulky Waters who love to spam Ice Beam.
Landorus@ Choice Band
Adamant | Intimidate
4 HP / 252 Atk / 252 Spe
- Rock Slide
- Earthquake
- U-Turn
- Superpower / Stone Edge
Bandorus shines in Tailwind, as well as being possibly the best partner for Mega Altaria. Intimidate bulks up Alt's defenses, while Choice Band is chosen over Scarf because of Tailwind. I prefer Stone Edge over Superpower to get some surprise KOs on Thundurus who aren't running as much bulk as they should, but each to his own.
There, those are the Pokémon that I think will work best as teammates for Mega Altaria. You may have noticed that I brought up Tailwind a lot; that is literally the only way you should be using Altaria, either as a bulky setter or abuser.
