Everytime you summon a demon, you need to be casting empower. Before you engage, you need to cast empower.
Thel'kiel's is very very hard to set up in PvP as casters are really punished right now, but, the way to go is:
Empower > Doom, bolt till 5 shards, Doomguard, Gul'dan, empower, boltx2, dreadstalkers, empower, bolt and gul'dan till you only have a bout 3 seconds left on imps, empower, thel'kiel's, implosion.
If you did this correctly, then you should have used consumption on ~9-11 demons, with a 4-7 imp implosion. Mind you, this is a perfect situation where they just let you free cast, so chances are you'll only get to do 75% of this.