I just ended act 2 by killing arishok. But i had great dificulties doing so, when i looked it up on the internet it was 1on1 vs just arishok, for my part i had to fight ~30 mobs at the same time (all the mobs at the same time)
I also had my whole team there?
Is this a bug?

No its not.
You face him 1on1 if you play rogue..
As warrior you will face hes entire army;)

Thats not true.

You fight his guards every time. Then you 1v1 him if Isabella comes back and you refuse you give her up. If Isabella doesn't come back (If even possible) than I think you can use your party.

Might as well add that if Fenris is in your party he will propose a 1v1.

It is actually, because you wont meet Isabella when you play as warrior;)