Most BG fans figure this out eventually anyway, but I figured I'd just do a quick writeup of it myself.
(This is done on BG1Tutu, but some variation on it should work on all versions of Baldur's Gate)
Alright. For this trick, we will need to head to the nameless area South-by-southwest of Beregost. You'll encounter a large group of gnolls here and Drizzt will ask you for your assistance in defeating them. Accept and watch him beat the shit out of about 15 gnolls in as many seconds. Yeouch. DO NOT talk to him again after the battle or else he will leave.
Since we definitely don't want to try and tangle with him in a straight fight (and his Magic Resistance is too high for any magical shenanigans), we'll have to exploit his AI to kill him.
Move your party on to this little dock in the north end of the area:
Move one character (not your main!) off the dock, and position two of your characters (also not your main) at the edge of it, spaced just close enough to bar people from getting at the others hiding behind them on the dock. Move your main character up right behind them and boot them from the party; they will stay right where they are. This will be our roadblock.
Make sure the people behind the "roadblock" all have some kind of ranged weapon - halberds, bow and arrows, quarterstaves, slings and rocks, whatever. Anything with a bit of range will work, and no weapon proficiency points are needed - you just need to be able to use them as weapons. Melee weapons are preferable as you don't need to worry about exhausting your ammunition supply; you can loot plenty of halberds from the dead gnolls below if you need them.
Now move your outside NPC back down and attack Drizzt to get him to follow you. Lead him up to the dock. If you're playing on Tutu you should be able to move them to safety behind your "roadblock" without letting Drizzt in; if not, they'll have to bite the bullet and let Drizzt kill them.
Now, have your characters behind the "roadblock" attack Drizzt with any ranged weapons at their disposal. They should be able to freely attack Drizzt and not be attacked themselves - he will not target your two "roadblock" NPCs since they're not in your party and therefore aren't a 'threat' to him.
Owing to Drizzt's absurdly high AC your characters will have to roll twenties to hit him, so this will take quite a while. However, he is a complete sitting duck if you've set this up right, so after 10-15 minutes you should wear down his 96 HP and kill him.
Eventually you'll see a message similar to this one pop up:
And your remaining characters will get a large chunk of XP. You can also loot his body for the best equipment the game has to offer: Two kickass magical scimitars and a very good suit of magical chain mail.
(Frostbrand also has an undocumented effect of granting +50% fire resistance)
Note that Defender can only be wielded by characters of Good alignment, so don't plan on using it if you're playing an evil character.
Oh, and be sure to hit up a temple afterwards and buy your lost Reputation back!