I can't beat the conspirators. The problem is that it places you in a cramped room with 7-8 to kill.
What is the easiest way to do this?

Make sure you have quen on at all times ( i hope you upgraded it to deflect damage but even if you didn't it will atleast protect you)
and there's a small passage before you enter the room . you can drink potions there.
Have bombs like Dancing Star , Grapeshot and Zerrikanian Sun in your pockets and dont shy from using . Because its such a closed space enemies are all clustered together, so bombs do a lot of damage.

This is basically what i did and didnt really have too much difficulty in killing them.
Hope it works for you.

okay thanks. I have upgraded Quen once and I guess I should make use of bombs more often.

You can also inform Dethmold that you know where the conspirators are hiding before going there. He will lend some of his men that will appear to fight with you when the show begins.

I done it now and I didn't need Dethmold's help. I don't why I didn't think of using bombs before. It made the fight easy.