Siege of Dragonspear Chapter 9 Boarsyr Bridge.Towards the end of the chapter you have to speak with Khalid and decide whether you will fight for the fort or surrender.
This is a tough battle, Khalid will give you the opportunity to return to camp and summon help from the flaming fist.
Use this opportunity bring Baeloth and Edwin in your party and ensure edwin has fireballs charged.
Now just use your mages and hit them with fireballs, this devastates them and wipes most out.
Now reload and get the achievement for :
Solution for Inside Job In Baldur's Gate and Baldur's Gate II: Enhanced EditionsAlso the 2 achievements for forgingitems from voidstone and dragonscale can be earned in the fort