Nairn is a Humanoid, and I wanted something that doubled for both rounds. So, Harbinger of Flame really shines here! Start with the classic Immolate and Conflagrate combo. Then just spam Burn till he's down. You want to save your next combo for the mechanical. Nairn just changes the weather and shouts his name doing damage, but its weak against humanoid, so just harbinger rape him!
Stormoen is Mechanical, and nothing is nastier to a mechanical then elemental fire damage. So start with immolate, then conflag if you can. If the harbinger goes down before either, use the Crow’s or Raven’s Darkness to change the weather and follow it up with darkness. then its just Alphastrike spam till he's dead.
Summer is Beast. By the time he joins the fight, your crow should have the advantage. Start with whatever isn't on CD and chain the abilities till the Crow goes down. Hopefully you can get the Dodge on cd so its not up for Sentry’s Call Lightning! If you have Son of Animus out, make sure you use Siphon Anima when the Dodge is cooling down. Either way start with your Extra Plating
to mitigate some damage, then just Laser spam with Sentry until Summer GETS OWNED!