I am level 22 Elven Ranger. I specialized in longswords 2 stars and crossbows 2 stars and a few others but those were the two I used. I got a wicked crossbow and longsword setup and usually favor both depending on situation. I have healing/buffer spells when needed up to level 3.

Is there anything I should do other than buff up my strength and haste with potions. If anyone has faced this guy any ideas for his weakness are helpful.

What weapon type with melee is better at damaging him?

Don't bother buffing before you enter the room with Belhifet. You're automatically dispelled. He has a lot of immunities. Your best bet is the best sword you have, lots of healing potions, and oil of speed. Chug the speed potion, and go after big B. You want to do as much damage as you can as quickly as you can. Solo is going to be rough because you're also going to have to cope with the golems. Just watch your health and keep chugging healing potions.

Just remember that he is 100% resistant to fire, 50% resistant to cold, 50% resistant to missiles, 50% resistant to magic, 25% resistant to slashing, crushing and piercing. He has no resistance to electricity or acid. Also remember that using certain 'magical' items will have to get past his magic and spell resistance. So any melee weapon with electrical damage would seem to be the way to go, but not strictly essential.

So with 50% resistance to ranged/missile weapons, will running away hasted while shooting him work if I alternate between melee and ranged?

Do you mean kiting? It might be a valid delaying tactic, but you'd probably be better off leaving the running until you need healing or some other task. As Coelocanth said, you want to pile on the damage quickly, and using missiles is not going to do that.

Why do you need to do the damage early on? Is there a certain thing that happens if you don't kill him quick?

Edit-Im also going to guess he can see through hide/stealth. Oh well, I'll give it a try.

It's more that he does a lot of damage and doesn't tend to miss often. As well, he'll later hit you with Flame Strike or Fireball (or something similar).

I bested him with melee and ranged combat! It seems that I did enough damage with crossbow to take him down a bit. From then, I managed to dice him up.

I should have made a fighter/thief however. If anyone knows if he can see through invisible or be backstabbed, I might try that. It would have saved time if I could get one wicked backstab in there. Either way, a great game.

Thanks to everyone for kind help. And if anyone knows he can see through that be great to try another build! :)

I just took a look at his creature file, and he has an equipped ring that gives him immunity to spell levels 1-9, and confers detection of invisibility on him. Also, if his action list is empty and he cannot see the PC, he will move to the PC anyway!

All the enemies (Behilfet, Cornuguns, Golems) can see invisible. But they tend to prefer going after visible targets if a visible character is closer to them than an invisible one. None of them will melee attack Sanctuaried priests, but Behilfet will cast his spells on anyone in range he sees, regardless of spell protections. Its a good reason to buff up on Resist Fire, so Behilfet's fire-based spells heal instead of do damage.

Behilfet's spell immunity makes it quasi-impossible to make any type of spell cast at him stick. I did one time finish off a Near-Death Behilfet with a Power Word Kill, but that was after exhausting every other spell I had memorized, all to no effect.

There is a pretty detailed account of how to beat Behilfet with a 2-man Mage / Cleric team here: http://www.gog.com/forum/icewind_dale/boss_killing_strategies

Probably it depends on difficulty level and whether expansion is installed or not, but the first time I finished it on normal and without expansion, a Chromatic orb went through his spell resistance and paralyzed him (lucky, I know)