I'm thinking this for my Houndoom:

@ Wise Glasses
Rash - Flash Fire
252 SpA / 252 SpD / 6 Att
~ Nasty Plot
~ HP Flying/Psychic
~ Flamethrower
~ Dark Pulse

I'm kind of new to this, but I think this may work.

Ground works also, but fighting would still be better as Tyranitar poses more of a threat than Heatran.

I 2nd what shina says, but also add HP ground, one kills Ttar one kills heatran, whichever is the worse threat.

Since yours is rash nature, if you have a 31 attack IV you can also use focus sash+reversal or sucker punch+life orb for some mixed options.

I have a houndoom and always gave it attacks that counter it's weaknesses.
Houndoom is a fire and of course weak to water, so have it learn thunder fang from the move tutor.
The biggest selection is the dark type attack since there two great attacks houndoom can learn Crunch and Dark Pulse.
Crunch can cause a defense reduce while Dark Pulse causes a flinch.
The strongest fire attack that doesn't cause a self-reduce and 100% chance of hit is flamethrower so that's a good choice.
Finally the fourth depends on strategy and you can have any attack you think is good.
I never gave it a fourth move and just kept Nasty Plot, but Embargo and shadowball will work fine. (Shadow ball is strong against Fighting pokemon and houndoom is weak against fighting attacks.)

So if you want a defensive moveset more than pure attack power then I would give it these attacks.

why don't you try

@ focus sash
flash fire hasty nature

nasty plot
flamethrower
hidden power fighting
pursuit/dark pulse

EDIT: 252 spa 126 hp 126 speed ?