aww, come on, Parry + Throw the Gauntlet - and for 20 seconds the Arishok can't hit you, at least on lower difficulties (unless he impales you, that is)
lvl15 is odd though... out for curiosity, what level were you in the deep roads?
you say
"I wanted to see what reacion i did if I let Isabela take the relic back, but whatever option you do? You STILL fight the Airshock. ."
does that mean you have a save before or during To Catch a Thief (since this is the quest where you make "decision" about the relic)? Because if so, you can still go shopping for potions and grenades after the quest and before you meet Aveline at the compound
The team fight is indeed rather easy, especially if you disable the mages first (freeze/petrify/stun/whatever and kill before they start casting their electric bubbles of instant death). Fun fact: Vendetta can teleport you to the balcony

With the duel my biggest problem is that I always forget to stock up potions and grenades beforehand...otherwise it's just a test of patience: hit and run, like many people here have said.
/*Combustion grenade to stun (Miasmic flask works too when he has low health) ->*/ Mark of Death -> Assassinate -> Twin Fangs -> kiting ->kiting -> get out of his way!!! -> kiting-> kiting ->Assassinate cooldown ended! -> drink potions, better when he's charging -> repeat
Since you have a good amount of stamina potions (11, wtf? I have 4 max usually, Varric is such a drunk), during kiting you can add some of Vendetta -> Throw the Gauntlet -> hit him repeatedly for 20 seconds -> run like hell -> repeat 1
/*This is considering you have at least one of high-damage rogue skills (Twin fangs, Vendetta, Assassinate). If not...I'm sorry to say this, but your build is probably wrong

*/
But you should not listen to my rambling, the only class I
actually had the patience to finish this kitefest without turning down the difficulty was mage...I know: I am not basalit-an, I am unworthy.