Pokémon X and Y Walkthrough, Pokémon Move Sets: Houndoom
(Please note that the recommendations below are largely made for in-game play. A Houndoom in a competitive environment will likely make use of different moves to accommodate for smarter, more adaptive opponents. Got a different kind of pooch in your team? Feel free to share it in the comments.)
The pokémon that launched a thousand complaints from overly-zealous parents, Houndoom is an undeniably cool-lookin' critter. Though it's perhaps not as useful as it used to be Houndoom is still a great addition to any team, and its new Mega Evolution only bolsters Houndoom's overall potential. You thought two horns was cool? Try eight.
Type: Dark / Fire
Houndoom begins as the diminutive, grouchy Houndour. Level Houndour to 24 and it will evolve into a Houndoom. You can then further evolve Houndoom into Mega Houndoom via Houndoominite.
Dark, Fire, Ghost, Grass, Ice, Steel. Immune to Psychic.
Fighting, Ground, Rock, Water.
A sweeper? You betcha. Houndoom's greatest strength is its Special Attack, with a heaping 110. Speed comes next at a respectable 95, with Attack in third at 90. Special Defense isn't bad at 80, HP is meh at 75, and Defense rates a paltry 50. It's this last one that is most troublesome, and even Houndoom's Special Defense is not great enough to ward off most super effective attacks.
Mega Houndoom perfectly compliments Houndoom's strengths. Its Special Attack rises to 140, while its Speed, the biggest problem most of the time, jumps to 115. Defense and Special Defense also get bumps, allowing Houndoom a decent boost to survivability, but you still want to treat this sucker as a glass cannon.
Early Bird: The pokémon will awaken from sleep status in half the normal time. This would be much more useful if Houndoom wasn't such a frail pokémon, as it could put Rest to much greater use. As it is... not so much.
Flash Fire: if the pokémon is hit by a fire-type move, its fire-type moves will be boosted for the remainder of the fight rather than taking damage. In essence, Houndoom is utterly immune to fire-type moves. Most opponents won't bother to attack Houndoom with fire-type attacks, so this is best used if you can predict a Flamethrower or Fire Blast or what have you and switch into Houndoom. Better than Early Bird, at least.
Unnerve: The pokemon's oppoent will be unable to eat berries. Quite incidental, this, and not that great. Unnerve is a hidden ability.
Solar Power: The pokemon's Special Attack stat is raised by half under the effects of sunlight, but it loses a small chunk of each turn it is exposed to sunlight. A fantastic ability so long as you ensure that your Houndoom goes first each turn. Solar Power is only available to Mega Houndoom.
Houndoom doesn't quite get the range of attacks you might like for a sweeper, so you need to make do with sacrificing one turn to boost its damage output. Nasty Plot is the way to go in this regard. Once Nasty Plot is in play you can use STAB moves to blow away the competition, namely Flamethrower and Dark Pulse. Hidden Power is pretty popular with the online crowd, but I'd suggest Sludge Bomb to wipe out fairy-types instead.
A Mega Houndoom has a little more range, thanks to Solar Power. Either swap it into sunlight or give it Sunny Day to deploy sunlight on its own. Then use Solar Beam to plow through the competition one at a time. Even if you have to swap to a different move to cover for resistances (again, Flamethrower and Dark Pulse) you'll still get a significant boost to your damage output. Will-O-Wisp is a possibility if you want to offset some of your opponent's attacking power, though I prefer strictly offensive moves.
EV / Super Training
Your standard Houndoom is best given ample amounts of Special Attack and Speed EVs. Speed is of particular importance, as most Houndooms cannot survive even a single attack, especially in online battles. If you opt for Mega Houndoom you can swap out the Speed EVs for something more defensive, possibly split between Defense and Special Defense (or straight HP); just make sure the remainder go to Special Attack.
Catching a Houndoom
Houndoom does not appear in the wild on its own. You'll have to get it by catching a Houndour, which appears on Route 10 in Pokémon X. You'll have to trade for it in Pokémon Y. If you want Mega Houndoom, you can find Houndoominite near the eastern entrance of Route 16; look for a rock formation circled by two skaters, and check the formation between 8 and 9 PM for the Mega Stone. You can only find it here in Pokémon Y. (See what they did there? Tricksy.)