What's the nbest tactic to destroy the Demon inthe Planar Sphere? I had nalia but she left because of her father...I'm left with Anomen and Jahiera as spell casters... Help!

I did that board today and found it pretty tough too. What level is Jaheira at? I used her to summon a couple of fire elementals who distracted the demon while the rest of my party hit him with arrows and stones while Valygar went to attack armed with potions and a pair of Katanas.
Someone else suggested using silence on him too. Apparently that stops him from doing very much damage. Just cast it in the demons vicinity just before you actually see him. I don't know how well this works though.
Helpful I hope.

you lost your mage, that's a bummer.

I had to reload a couple times when I first did it, but eventually, I just unloaded everything I had on it. Mostly, I used a -lot- of summons to distract it while my ranged fighters did their thing. I mean, the demon got through the summons pretty quick, but I kept them coming so it bought me a lot of time.

Probably the hardest bit here is which Demon are you getting stuck into? From memory, there are three of them, and two of them are very difficult. If you're going after the one surrounded by mephits, go another one. I can't remember his name, but he should be in the upper center of the map.
What levels are you? Because that will significantly change your strategy. And who else is in your party?

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The meek shall inherit the Earth. But the strong shall reclaim it as their own.

This demon is tough. Here are some
spells to cast on party members that
will help:

Resist Fear
Free Action
Chaotic Commands
Defensive Harmony

and summoning elementals or conjure
animals will also distract the demon.

Oh and Haste might help also, and
mass cure too.

Someone suggested (in another thread) that casting Silence on the demon before you attack him makes the fight very easy.

I just had to kill all the demons on that screen and take their hearts, you never know when a demon heart might come in handy. I only ended up with 3 of them but it is fun killing demons. I just swarm them with my 4 hasted fighters holding 2 weapons each I had one character die on the last demon, but a harpers call took care of that. long live kensais

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"They come in the dark. Always in the dark..."