I think I'm at the very end of the game but I'm having problems with the Slayer & bunch of other creatures that are aiding it. Is there any pointers anyone could give me on how to get past these guys?
Thanks in advance folks.

Right - the way I did it:

First, before inserting the last tear of Bhaal summon a couple of scary monsters. I used Fire elementals as they are immune to the spells I was to use later...

Then cast haste and chant. Use any spell protections for mages, and cast stoneskin on anyone with it memorised. Make sure everyone has the stuff they need equipped.

Place the last tear of Bhaal in the slot. When the mosters summon Use the summoned creatures to distract the demons and pile on your own big fighters to the Slayer. You should have a Time Stop Scroll or two kicking around by now so have your mage use it and cast Abi Dalzims repeatedly. This should severely weaken a few of the demons despite high magic resistance.

Keep pounding away at the Slayer and he will probably summon a Simalcrum. When he does this he will use the time in which you bash that to escape to the bottom of the area. While he is gone finish off the demons.

Now recast your haste and chant spells and move everyone to the bottom of the screen except for weakling mages etc. Just hit him repeatedly. Move the mage in afterwards so they don't get in the way and cast Breach. Then if he is still alive the highest level spell protection remover you've got, followed by magic missiles, acid arrows, and flame arrows.

It shouldn't take too long to finish him off when you get this far.