Yeah each battle horror is worth 4000 exp points if you can take them down, here's how I did it.
Place your group in the room before you see the first of them, ensure only the people in the heavy metal are up front becuase unless your super hard they'll break you. Choose the strongest (Best AC) in your group and inch forward bit by bit, you have to get each of them on their own, it's impossible otherwise. When the first guy enters the room get all the warriors to jump on him from all side (He can't defend in every direction) If you have a haste spell or The Oil of Speed - use it. The more hits you get in the easier it will be. If your character that got this horror to chase him takes a lot of damage, pull him out of the fray. If the horror follows him wander around the group, the horror might follow if he does your group will land plenty of hits before he chooses another. Then pull the character leading him around back into the fight.
They ain't imposible I've killed 6 between BG and TOTSC.

Good Luck.

Thanks for the advice. I will try that.

I am getting so frustrated with the mines in Chapter 4. I get to this wizard on the bottom, but he has two Horror things with him that are like impossible to kill, and they kill your guys in like less than a second. Ive killed the wizard but I cant kill the two Horror things. Sorry if you dont know what im talking about. I used the wand of animal summoning 3 times and they just kill all of them. I am not in the mood to loose 5 of my guys and still have two of them running around killing me with there flame swords. How do you kill them? If you can because from my expierence they have allot of hit points.

There's a much easier way
Send out a hidden thief and let him search for traps
there a 2 traps one for each battle horror disable the traps and the batle horres won't appere

Or you could just extract the creature files or whatever for them, make them wimpy, and then plop the files in the override folder.

Here is how I did it
Fisrt send in your two most heavily armoured characters, cast as many spells or scrolls on them as possible, sheilding spells and strength potions, then when they are attacking send in the other guys and use a mage with a wand such as frost or some heavy spell
4000 experience points is worth the effort