how am i suppose to kill firgraag???

There are probably around 100 threads on Firkraag on this board so I suggest you check them all out.
The main things to keep in mind are:
Make sure you are of high enough level
Summon some creatures as wing buffet fodder (a Pit Fiend is nice )
Have lots of healing potions ready
Almost everyone on the board has a different tactic (use magic only, use fighters only, use a monk) but I think you should make that out for yourself.

LOL, just check out my wronger than wrong posted message below this one (temperairily).

Use the search button with Firkraag as the keyword and you'll find the info you're looking for.