Some sets with calcs and alternatives.
Mega Houndoom (Sun Nasty Plot)
Houndoom @ Houndoomite
Ability: Flash Fire (Solar Power)
EVs: 4 HP / 252 SpAtk / 252 Speed
Nature: Timid / Mild
- Nasty Plot
- Fire Blast
- Dark Pulse / Sucker Punch
- Solar Beam / Hidden Power Rock / Hidden Power Grass / Substitute
If sun is up, Mega Houndoom boasts one of the hardest-hitting special attacks in the game:
+2 252 SpA Solar Power Houndoom Fire Blast vs. 252 HP / 252+ SpD Thick Fat Snorlax in sun: 288-339 (54.96 - 64.69%) -- guaranteed 2HKO
+2 252 SpA Life Orb Mewtwo Aura Sphere vs. 252 HP / 252+ SpD Snorlax: 390-460 (74.42 - 87.78%) -- guaranteed 2HKO
This is a SUPER EFFECTIVE hit from a MEWTWO vs. a RESISTED hit from a HOUNDOOM on one of the best fully invested special walls in the game (160/110 Special Defense). Granted, it is at +2, but the chance of having such immense power may be reason enough to run Drought or Sunny Day support on a team to give Houndoom a chance at shining. Because Fire Blast vaporizes things even when resisted, everything else revolves around coverage. Dark Pulse is Solar Power-boosted and very, very powerful, but an uninvested Sucker Punch still has a chance to fight back against faster Pokemon such as Mega Gengar (regular Gengar is outsped and crushed by Fire Blast, even if it tries to Mega Evolve that turn), and Fire Blast obliterates everything anyway, making mildly powerful physical priority something that might be appealing. The last spot is mostly filler, but getting through the Pokemon who quad resist Fire Blast or have Flash Fire may give it problems. And speaking of Flash Fire, if you manage to switch Houndoom in on a Fire-type move before Mega Evolving, this is about the equivalent of a Solar Power boost to Fire Blast. Something says goodbye.
Mega Houndoom (SunnyBeam)
Houndoom @ Houndoomite
Ability: Flash Fire (Solar Power)
EVs: 4 HP / 252 SpAtk / 252 Speed
Nature: Timid / Mild
- Sunny Day
- Solarbeam
- Fire Blast
- Dark Pulse / Sucker Punch
The old school classic returns! Although inferior to the other set due to less power, Flash Fire on normal Houndoom can still make Mega Houndoom an absolute MONSTER, and being able to set up Sunny Day for itself with slightly improved defenses can allow it to still wreak havoc, now with a more reliable Solarbeam to boot. SunnyBeam has always been a weird strategy because its users took hits like wet tissues and it was almost completely wrecked by auto-weather in Gen V, but it's hard to resist the potential of OHKO'ing a good part of the metagame with just a couple of attacks.
Mixed Attacker
Houndoom @ Life Orb
Ability: Flash Fire
EVs: 252 Atk / 4 SpAtk / 252 Speed
Nature: Hasty
- Pursuit
- Sucker Punch
- Fire Blast
- Crunch
The Gen V standard that became one of the strangest high-risk, high-reward Pokemon in UU. With good prediction, any one of Houndoom's three physical 80 BP STABs could put a good dent in your opponent no matter what he chose to do: switch, attack with a physical move, or attack with a special one. It remains viable in this generation, especially because many will be expecting the special attacking Mega Houndoom only to get hit with a physical set instead. It makes Ghost- and Psychic-typing on many Pokemon a liability, but predicting wrongly will almost certainly end up with a dead dog on your hands.