This is Houndoom's bread-and-butter Nasty Plot set. Fire Blast and Dark Pulse are both used on this set to make the most of the excellent coverage afforded by Houndoom's dual STABs. Fire Blast is Houndoom's preferred sweeping move due to its high Base Power, while Dark Pulse hits the Fire-, Water-, and Rock-type Pokemon who resist Fire Blast for heavy damage. After a Nasty Plot, Moltres, Arcanine, and Rhyperior are OHKOed, and Milotic loses upwards of 70% HP.
It may be odd to see Sucker Punch, a physical attack, on a Nasty Plot set, but the move is actually an excellent tool for Houndoom, both in assisting its own sweep and in putting a stop to opposing sweepers. Sucker Punch always OHKOes 4 HP Dugtrio after Stealth Rock, which means without Substitute and good prediction it cannot stop Houndoom's sweep. Sucker Punch also prevents Choice Scarf Rotom, most Alakazam, and some Mismagius from revenge killing Houndoom. Finally, even when Houndoom is not attempting a sweep, Sucker Punch allows it to finish off faster, weakened foes.
Flash Fire is used instead of Early Bird to boost Houndoom's offensive potential; for example, if Houndoom switches into Weezing's or Arcanine's Will-O-Wisp, the subsequent +2 Fire Blast will be able to 2HKO Chansey. Since Houndoom is using both physical and special attacks, a Timid nature isn't optimal. Hasty is advisable instead of Naive because it makes more sense to lower Houndoom's already dismal Defense rather than to sacrifice its decent Special Defense. Also, Houndoom's resistances are usually seen in the form of special attacks (notably Shadow Ball), and higher Special Defense means it is easier to set up on weak special attackers, such as Weezing with Sludge Bomb.
The EVs may seem complicated at first glance, but fulfill specific purposes. 224 Speed EVs and a Hasty nature are used to outpace Timid Rotom along with all slower Pokemon, including Timid Moltres and Venusaur. 100 Attack EVs with a Life Orb are necessary to ensure Sucker Punch always OHKOes 4 HP Dugtrio after Stealth Rock damage. The remaining EVs are put into Special Attack to maximize Houndoom's offensive punch. If you want Houndoom to tie with opposing base 95 Speed Pokemon, such as Leafeon and Drapion, you can shift a few EVs into Speed; however, this isn't the recommended spread since Houndoom needs as many EVs as possible to spread between its attacking stats.
Houndoom is hard to counter perfectly because of its high damage output, but there are a few Pokemon who are good switch-ins. Milotic, particularly specially defensive variants, can take a boosted Dark Pulse at full health and KO Houndoom with Surf. Physically defensive Milotic should be wary of taking too much previous damage, though, as Dark Pulse deals 63.4% - 74.8%. Chansey can stall Houndoom out with Toxic and Wish / Protect as long as Houndoom doesn't get a Flash Fire boost. Note that if Houndoom starts using Nasty Plot on Protect, Chansey will probably have to switch out a turn or two before Toxic kills Houndoom (perhaps to a Pokemon who resists or is immune to Fire, or a sacrifice). Azumarill resists Houndoom's primary STAB, doesn't lose too much HP to an unboosted Dark Pulse, and can use its priority Aqua Jet to OHKO Houndoom before Houndoom can attack again. There are three Pokemon in UU that resist the combination of Fire- and Dark-type attacks: Houndoom, Blaziken, and Thick Fat Hariyama. Against other Houndoom, the Houndoom who has more boosts under its belt wins with Dark Pulse. Blaziken is frail and doesn't appreciate eating Fire Blast too often, but its Vacuum Wave makes quick work of Houndoom. Thick Fat Hariyama with a specially defensive spread is an excellent Houndoom counter. Its "weakness" is its low utility outside of this role, as other common Fire-types in UU either have strong physical attacks (Arcanine and Blaziken) or have a super effective STAB attack (Moltres). Faster Pokemon who can take a Sucker Punch, such as Tauros, Scyther, and Ambipom, are good revenge killers.
If you're not keen on using Sucker Punch on this set, there are some Hidden Powers that may interest you. Hidden Power Flying hits two of the three UU Pokemon who resist Houndoom's STAB combination (the last, other Houndoom, is not a big concern) super effectively, but has no utility outside of that. Hidden Power Grass is one-sixth more powerful than Dark Pulse against Water-types, but even so, cannot OHKO Milotic. Obviously, if you use Hidden Power in the last slot, Houndoom doesn't require any Attack EVs, so a simple 252 SpA / 4 SpD / 252 Spe spread with a Timid nature works best. A Modest nature also becomes more viable with Hidden Power Grass, since Houndoom can then OHKO Bold Milotic with a bit of residual damage.