Grachus Options

[Lemernis]

I've tried using Dispel Magic but Grachus doesn't seem to skip a beat. I do my best to get the party to try to do some damage, but either Grachus or the Moon Horrors start whupping on them. They are unable to do much to Grachus, and eventually everyone in the party falls one by one as their spell and potions buffs wear off. As I mentioned, I will not do any appreciable damage with ranged attacks from this party. They pretty much have to melee.

Painful way of suicide. Even Sarevok would bite the dust if he faced Grachus alone.

... You should run a lot, and let... other non-party members do the dirty (melee) work... You might be impatient in battle, and don't run enough, do you?

Yes, absolutely. I do rotate characters into face-to-face combat, and when the one fighting Gracgus directly is badly injured I move that character away to heal via potions or cure spells.

Although I haven't played the Tutu version yet (except some testing), it's hard to believe that Grachus has too many abilities left after fighting Talibehius, Tergius, the Dwarves and Damien...

... Grachus has a limited number of Contingencies, Tattoos and Necromantic Abilities/Spells. He will use them up while fighting the aforementioned allies of you. He shouldn't be too strong when you lure him to Damien... Somehow you are silent about Damien -- did you make an attempt with him at all?

Well, I have a save where I had led Grachus and his summons directly to Tergius. And as I mentioned Grachus killed Tergius. I hoped Grachus would be substantially weakened after that encounter, having cast most of his spells by then. However he is uninjuredand seems to have plenty of destructive power left. And when I load the save at that point, and lure Gracgus to the dwarves, Grachus simply summons a fresh batch of Moon Horrors. Grachus and his Moon Horrors then eventually prove too much for the dwarves and my party.

Is Damien one of the dwarves?

***

Near as I can tell, the main problem I'm having here is that my party is heavily dependent spellcasting to win (especially with the much more difficult SCS battles, TGC notwithstanding). That is the challenge I intended to set up for myself at the outset with the party composition of two mages, one tank, a thief, a full cleric who supports the party with healing, item identification, and divine spellcasting, and a cleric-thief who suits up for melee. But here, with Grachus and his Moon Horrors, this party is facing SoA/ToB level opponents who appear to have magic resistance (is this correct?). And I don't have the BG2 spells such as Breach, Spellstrike, Lower Resistance, etc., that would allow me to defeat them with magic.

[Baronius]

Is Damien one of the dwarves?

Damien is human, a member of Amn's economic and military elite. The Grey Clan kidnapped his wife, so he came to to FAI to free her. You can find him on the northeast corner of the map, please see Map4 here: http://greyclan.blackwyrmlair.net/ep1/maps.html

Near as I can tell, the main problem I'm having here is that my party is heavily dependent spellcasting to win (especially with the much more difficult SCS battles, TGC notwithstanding). That is the challenge I intended to set up for myself at the outset with the party composition of two mages, one tank, a thief, a full cleric who supports the party with healing, item identification, and divine spellcasting, and a cleric-thief who suits up for melee.

Facing challenges often requires unusual, non-standard approach. And it's common that you have to use every possibility to win. This can be your solution here.

But here, with Grachus and his Moon Horrors, this party is facing SoA/ToB level opponents who appear to have magic resistance (is this correct?). And I don't have the BG2 spells such as Breach, Spellstrike, Lower Resistance, etc., that would allow me to defeat them with magic.

In your case, the non-stardard approach means that you don't rank opponents by power in this way (ToB/SoA levels), because it is known that traditional tactics won't help here. But there is always a solution. You need the support of all allies. And even at this point, no one (not even myself) knows which combination is the best if it's not tried. For example, cmorgan's idea about moving counterclockwise on the map might be better than the usual one (clockwise), I have never tried it. However, one thing is sure, you need all allies. Now that you know where Damien is, you have one more ally. Check the map I linked to, there is Talibehius as well. He has a leather armour, so he won't stand for too long, but his hits are effective. Additionally, if Tergius was alive, he would keep healing Talibehius even from big distance.
(It might not be the best feeling that it's not your party that wins this battle but other allies, but it still means you can advance in GCE1's plot with such a party.)

All in all, you've to mobilize every ally, and every character's power (if/where applicable) to win this battle with such a party. When you see that Grachus is weak and starts losing hit points, don't let your ally give the final hit. Provided you want the 18,000 XP. (Melf's Minute Meteors? I haven't tried them on Grachus, but without magical protections, he should be more vulnerable to such attacks as well.)