For this achievement, in the last chapter, when you enter the church in the undercity to fight the final boss, you need to go in with five companions, then during the fight, let the bosses kill them. Once they're dead (their profile picture is greyed out), kill the bosses to win the game. Achievement should unlock right away. Be aware that playing in story mode will prevent the bosses from killing your party, so you'll have to play at least in easy mode.
If you're struggling for this achievement, or you are just cheevo hunting and don't mind cheating, there's a rather cheesy way to do this by using console commands. Close the game, open Windows Explorar and navigate to the folder C:\Users\[YourUserName]\Documents\Baldur's Gate - Enhanced Edition. There, open the file Baldur.lua with Notepad and add a new line at the end of the file, then type:
SetPrivateProfileString ('Program Options','Debug Mode','1')
Save and close the notepad. This will enable console commands in the game. Now go back to Steam, open and continue your game. When in the final boss room, take your character to the lower left corner, then move your party onwards. Go on through the dialogue, and once the battle starts, let the bosses kill your party (remember, don't use story mode). Then, move your character so that the bosses are in your line of sight, point your mouse at a boss and press Ctrl+Y, which will instantly kill the boss you're pointing at. Repeat until everyone else is dead. Achievement and final cinematic should pop right away.