Which is the best strategie to kill Samia and her group?
Thanks.

Just the same strategy you use for all groups: whatever works best for you. I don't think there are any strategies that are perfect for just this one group. So just do something along the lines of taking all spellcasters out with Insect Spells, Silence and/or magic missiles so that they can't get any spells out. Then hack up the fighters. A Spell that works really well on almost every adventurer group you encounter (and on other enemies as well for that matter) is Holy Smite. It's cast relatively fast and does a lot of damage especially if the cleric has a high level (however be careful that you do not harm any evil aligned characters in your own party)

Tricky to pull off but has a GREAT, I mean 'GRRRRREAAT' effect.

This trick requires you to do absolutely NOTHING afterwards!!!

have someone standing near the coffin with the dragonslayer.

Then let a mage cast GATE into the part where samia and her crew appear

before the incantation is complete - grab the items and samia should appear where the GATE is being casted. (timing is crucial)

result: samia turns evil - pit fiend gets casted just seconds before or afterwards

then SHUT THE DOOR (yes you must do this) and relax as the pit fiend destroys the whole crew

the pit fiend should kill most of the crew and if it doesn't then just finish the job, which should be easier now.

Or if you want the experience from killing the group / don't have the gate spell, throw an emotion spell into the room. They'll all go down, and you can harvest their bodies from there. Command / hold person whoever manages to make their save.

Well of course I happen to do it the third way eh..?
Well first of all, when you have collected all the goodies from the tombroom, equip everybody with closecombat weapons, cast protective spells for your party, cast som cleric spells to, be buffed out. Have a thief stroll in later, preferably invisible or running around hide in shadow mode for a nice backstab, position mages in the middle, least dangerous foes there. Have a good fighter or someone capable of dealing out a good wooping take Samia, shouldn´t be too hard, you should have pretty good weapons by now.
Cast quick spells at fighters and have a your thief backstab the mage real quick, ought to take care of him, then have a fighter meet the cleric and the big guy. One after another they will fall if you are prepaird and then its done. Just take out thoose mage and cleric fellows, they´re the most bothersome.

You can also walk in with one character with sanctuary and have them popup, hide a thief and summon all kinds of creatures you can, raise dead, air elementals, pitfiends cacao adn so on and move the sanctuary guy/girl out of there and let the rest of the party stand back firing away with distance weapons and spells..