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A nice tactic for Narcillicus encounter in Valley of the Tombs [BG1] [Spoiler]
Narcillicus, or rather the two Mustard Jellies he summons, is one of the more difficult challenges faced in Chapters 1-3. The Mustard Jellies have a high Magic Resistance, Piercing Resistance and ordinary weapons have little effect on them. They also have a potent offensive Breath Weapon and can put meleers in Slow-motion.
But they are each worth 2000XP and Narcillicus, who is much more easily subdued, is worth 1400XP with several nice Mage spells. So figuring this one out is very worthwhile even if it requires a Power Word: Reload or two. Even so a Level 2-3 Party may be stymied.
One way I recently discovered of finessing the encounter is as follows:
Before having a go at Narcillicus, head South and then East when you first encounter Cactus, clear out any bad guys who may have spawned near the entrance to the Ghast Tomb. You do not want them in your way when you return. Then head back to the area where Narcillicus is to be found ~approaching from his SE.
Choose the dialogue options which turn him hostile and results in the 2 Mustard Jellies. Ignore them for now. Concentrate all your firepower on Narcillicus who should go down quickly if you have good ranged attackers [high dex]. Now beat a hasty retreat back to the Ghast Tomb before the slow-moving Jellies can spew their Breath Weapon. Enter.
Handling that encounter is not so easy but after doing so one has plenty of time to prepare for the Mustard Jellies who are by now encamped immediately outside still waiting to devour you. But that is a good thing because you can straight-away engage in melee where they are not quite so deadly. Plus you can have a Cleric already posted nearby for Healing duties and perhaps a 3rd attacker armed with Quarterstaff, Halberd or other [Magical] weapon which has a longer reach. Not sure whether Flails have that feature.
Hope this helps!