Try this offensive moveset for Houndoom.
Flamethrower: Best fire type move Houndoom can learn.
Dark Pulse: Bedcause of Houndoom's higher Sp. Attack.
Sunny Day: Raises the power of Flamethrower and charges up Solarbeam.
Solarbeam: High power, good accuracy, charged by Sunny Day combo. Also can defeat all of Fire-types weaknesses (Water, Rock, Ground) leaving only Fighting types for Houndoom, which can be easily taken out by your power increased flamethrower.
Try teaching the Flamethrower, Sunny Day, Solarbeam combo to all Fire types that can learn it.