This build would have been hella fun with the old version of defiance...but the new version's not terrible either and doesn't promote quite the same level of suicidal gameplay.
Priority will always go to the aoe side of things if there's more then 2 guys, since every aoe that hits means -5% tohit and a chance to knock them down which is part of the survival factor, if they can't hit you....your buffed up regen with TotB can let you take a few hits without too much concern.
Group/pack:
Basic Opener - Throw Neutron bomb then dash in to drop irradiate just as neutron lands (its got a long travel animation afterall and its fun to abuse that), both proc knockdown which typically drops several dudes on their butts while the impressive -def debuff means OG and Death Shroud will have a much higher chance to hit and either knock them down again or stun lieus and minons.
"Rotation" - Not sure I can really call it that, its more hitting on aoe powers when they recharge....irradiate and neutron are staple attacks, Electron haze is a good backup if the other 2 aren't recharged yet (I actually don't use it that much but its nice to have) then I just lay into whatever the highest rank guy is with Midnight, Smite and Cosmic between aoe bursts. Dark Pit is a good "Oh shit" button to have, a wide area mag 2 stun that stacks with OG and leaves whole groups drooling for a few seconds.
Other Options - Just dive right in for Soul Drain and drop Atomic Blast...good times watching giant clusters just drop dead around ya. Void Judgement is the same deal but you don't need Soul Drain so its a good massive aoe attack when Atomic's not recharged.
Single Target:
Opener/Rotation - Cosmic burst to the face since its also a mag 3 stun meaning you can typically stun a boss before he even gets to react thanks to OG's stun, TotB to follow up for the -tohit and survival buffs (And a mag 3 fear) into Midnight n'smite and by then cosmic's usually recharged, repeat. The ST rotation is extremely simple like that and the cc you get is actually kinda crazy.