I've been using Mega Salamence for quite some time, and here's some thoughts. As memtioned above, "countering the counters" is the best way to go: clearing the path for Salamence to set up and sweep.
Mamoswine @ Choice Scarf
Ability: Thick Fat
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Icicle Crash
- Earthquake
- Rock Slide
- Superpower
Mamoswine has a huge Attack (130), and two great STAB attacks in Earhtquake and Icicle Crash, not to mention good coverage moves in Rock Slide and Superpower. keep in mind that it's Attack stat is only 5 short of base Salamence, and as fast as mega Salamence thanks to Choice Scarf, so he's a heavy hitter. SuperPower takes care of dark types, and Rock Slide is good against Fire/Flying types. Electric types are covered by Mamoswine also.
From a defensive point of view, both Salamence and Mamoswine cover each other quite nicely. Both are susceptible to bulky Steel types though. That's where Infernape comes in.
Infernape @ Focus Sash
Ability: Iron Fist
EVs: 252 SpA / 4 SpD / 252 Spe
Naive Nature
- Stealth Rock
- Taunt
- Fire Blast/Mach Punch
- Endeavor
Fire Blast will definitely finish off Ferrohtorn, and has a good chance of OHKO Meta Metagross. But use Infernape as a lead to set up Taunt or Stealth Rock first, and use Endeavor if Focus Sash is activated. Generally I have found that most people expect Infernape to be an all-out attacker, especially on a team with attacking options like Salamence and Mamoswine. However, seeing as Infernape is weak to water types, and that most water types carry an Ice Attack for coverage, you can either use Endeavor etc, or you could use....
Greninja @ Life Orb
Ability: Protean
EVs: 4 Atk / 252 SpA / 252 Spe
Naive Nature
- Ice Beam
- Grass Knot
- Scald
- Gunk Shot
Most Water types that are used competitively are heavy (thinking SlowBro, Kyogre), so Grass Knot will OHKO those that are. Gunk Shot is good against Azumarill, even on a Timid Greninja, though maybe a little esoteric? But it will take care of Fairy types for you well, which you'll definitely need with Salamence. Scald and Ice Beam offer good coverage. You may even want to add in Extrasensory instead of Ice Beam to take care of Fighting types. Yes, I know you've got Salamence to take care of fighting types, but those that carry Rock moves could hurt Salamence on the switch in.
Status affects Salamence in a big way, particularly paralysis and burn. Mamoswine can help counter Paralysis, but it also hates to be burnt, hence Infernape as your lead.