Or ask based lord
Recreant
Well, uh, since I was tagged specifically, I guess I have a reason for this...
To be honest, Mega Houndoom is one of those Pokemon that you seriously have to know what to do for it to work, and I'll tell ya, it ain't easy. Specifically, you need to know when to set up and when to send it in. aka, you don't want to click Nasty Plot turn one, and you don't want to take extra hazard damage if you don't need to. Most teams will have an automatic check to it whether they think about it or not (Azu, Keldeo, Scarf T-tar, Chansey, etc are all very common Pokemon seen on their respective playstyles), so they all need to be really weakened beforehand. It doesn't just work as a stallbreaker, and I really think people overhype that aspect (tears apart BOF stall when Chansey's down I guess, but with Sableye on the chopping board, that might no longer be applicable).
And best play styles/partners for it?
I generally use it with pretty heavy offense. As a trend, almost all of my teams with Houndoom seem to have Keldeo and Latias as partners. The three work so well together, Keldeo checks Hippowdon, Tyranitar, Landorus-T, Low Kick Weavile, and Heatran for Houndoom, while Latias is able to defeat Manaphy, Tank Garchomp, and Mega Venusaur while also providing Defog and Healing Wish support (for that last reason alone, I find it a far better partner than Latios). In return, Houndoom breaks down Mega Scizor, Ferrothorn, Gengar, Skarmory, and Mega Sableye for Latias and Slowbro, Celebi, and the Lati twins for Keldeo. Landorus-T is also a pretty obvious partner outside of those two, being able to check Talonflame, set up Rocks, and provide U-turn support, as KidMagic said. Toxicroak is also really neat, being able to switch into Azumarill's Waterfall and Keldeo's Scalds and set up against a lot of Houndoom's checks. (Generally, if using Latias / Keldeo / Doom, Sludge Bomb is better in that last slot, but if running Toxicroak, use Super Fang or Will-O-Wisp tbh)
so yea :>