For what it's worth, try disabling them instead of killing them.

EDIT: Ninja'd by Hickory, but if you don't want to use stealth, you can still stick to crowd control spells or wands.

I agree that stealth is probably the best way, since the guards may be able to save vs. spell against your fear or sleep spells. Since there's only one enemy to fight on the upper floor, you don't even have to worry about getting your whole party through--just be prepared to counter his spells in some way, as they make him pretty tough.

Just so you are aware: Sleep will be ineffective against the Flaming Fist at that stage of the game. Any creature with 4 HD (+3 hit points) is immune to sleep. And Fear (panic) allows a saving throw vs spells that completely negates the effect. Most FF Enforcers at that stage will have single figure saving throws.

One relatively fool-proof way of dealing with (small groups of) Flaming Fist officers is to use unarmed attacks. Punches will knock them unconscious rather than killing them. This stops your reputation from dropping, which in turn stops more dangerous groups from spawning in.

Also good to use is Algernon's Cloak, an item that can cast Charm indefinitely. You can pick-pocket it from a guy in Feldepost's Inn in Beregost.

Finally, you can take to the sewers and travel that way. No guards down there. Though it can be a bit of a pain to find your way around.

The other solutions work too, of course, but I find stealth is a bit difficult with a 6 person party. Though a good spellcaster definitely helps.

Now, if big groups of Enforcers are spawning in, your reputation is probably too low already and you won't have much success with the above tactics. The solution is pretty easy, though: find a church and donate money. The amount you need to donate varies: a lot (1500, I think) if your reputation is very low, less for moderate reputation, then the amount goes up again as your reputation exceeds average. But you only need to raise it a bit to stop the enforcers spawning.