Aha, Mulahey. He's also pretty tough. As before, buff your party members with everything. If you need to, backtrack up to the surface (there's an "emergency exit" in the cavern just outside Mulahey's lair you can use that will get you to the surface instantly) and buy potions and other things you might need. Using magic items and potions is essential sometimes...I know I usually hoard them because "what if"-situations arise.
I'm not really sure what tips I could give; buff your party. Make SURE that you use Jaheira's buffing-spells. A "Bless" before this battle is essential. It's a low-level spell which will remarkable increase your fighting ability. You may also try using her entanglement-spell on Mulahey to keep him from charging you.
I don't know how many npcs you are playing with, but as I usually play with six, when I encounter Mulahey I let two strong characters face Mulahey and another two guard the party's back when I enter the cavern. Mulahey is a cleric and will summon many undead skeletons behind you. Between these two pairs I keep a thief with a bow and a mage. Try to make attacks on Mulahey with magic missiles or acid arrows (or any ranged attack actually). For magic attacks, try to him him when he is casting a spell as this will (hopefully) make him lose concentration.
Yes, you can cheat your way to it. There are console commands for a lot of things in the game, but I don't remember...I STRONGLY advise against it, though. Baldur's Gate uses Infinity, a game engine which is very fragile. Tampering with it may result in a broken main quest and many other bugs. Besides, I think that Journal entries also play a part in the quests, and I do not know how to add those.
If the fight is far too hard for you, there is another cheat that you may use.
Quoted from CheatCC:
"Note: This procedure involves editing a game file; create a backup copy of the file before proceeding. Using a text editor, edit the "Baldur.ini" file in the game directory. Add the line "Cheats=1" under the "[Game Options]" heading. Then, begin a game and press [Ctrl] + [Tab] to display the console window. Type one of the following case-sensitive codes, press [Enter], then press [Ctrl] + [Tab] to close the console window and activate the corresponding cheat function:"
Then use this command:
This will "summon" (spawn) the game's most powerful fighter to fight at your side. Not as a party-npc, but he will defend you and attack your enemies. You may also summon more than one, and I don't THINK it will cause any problems with the game.
Anyway, try with the tactics I suggested first. Lower the difficuly level, perhaps? It's a very rewarding feeling, having beat a powerful enemy in Baldur's Gate.