This is the moveset that I use:
Darmanitan - Fire/(Psychic)
Ability: Zen Mode
Suggested item: Focus Sash
Suggested IV training: HP, Spd
Moveset: [Flare Blitz][Psychic][Focus Blast][Solar Beam]
Flare Blitz is a strong Fire-type move and takes recoil that helps turn Zen Mode.
Psychic gains STAB
Focus Blast is to take some Dark-types. Better you go attacking with Flare Blitz. If you and the oponent survive you may use Focus Blast.
Solar Beam is to to cover Water-, Rock- and Ground-types. You can start using the Solar Beam in Standart Mode. When you are charging the oponent may go using a Water-/Rock-/Ground-type move. You will have 100% chance to survive because the Focus Sash will defend you. But normally you will be the second to deal damage when you became Zen Mode and you will die but, with the Spd IV training you have a chance to be the first to attack. Now you can deal Solar Beam with high damage to oponent. Every times it works to me...
or if you can't/want get the Zen Mode
Darmanitan - Fire
Ability: Sheer Force
Suggested item: Life Orb
Suggested IV training: HP, Atk, Spd
Moveset: [Flare Blitz][Brick Break][Earthquake][Stone Edge]
Flare Blitz is a strong move plus Sheer Force and Life Orb will be able to K.O. everything.
Brick Break, Earthquake and Stone Edge is type coverage.