Hello,
first of all, I want to thank Sikret for this wellmade, challenging mod. It is hellish difficult, but also much fun, to figure out, what the right tacic for a certain foe is. However sometimes it is also extremly frustrating. Some enemies seem to be immune to everything, so I would like to know how to defeat them:
-The skelleton grandlord in the abyss, out of the planar sphere, he seems to be immune to everything.
-Quallos friend (in the sewers under the cooper cornet), is completly immune to my spells and weapons.

The both should be susceptible to +3 weapons and greater. Not sure about grandlords though, they might need a +4 weapon.

Are you sure that they were Grandlords and not just Lords? I have never met grandlords there. However it could depend on your level, if your level is high enough then you could maybe meet Grandlords instead of Lords. Grandlords can be damaged only by +4 weapons as I know.

Remember that you can get +3 weapons easily from the Enchanted Weapon spell. Additionally some summons (including elementals and mountain bears) strike as +3 or +4 weapons and can be useful against the Crawler.

What level are your characters? Like Vuki says you would only meet a Grandlord in the Planar Sphere quest if your protagonist has high xp.

Thanks for the hints My Party is about level 14. I think they were only lords, sorry. I will try it like this next time: haste, summon some creatures. Make the other skelletons and demon chase one hasted character and try to kill the remaining with the rest of my party, without loosing too much summons/stoneskin spells.
What tactic would you suggest against skelleton lords?

For a level 14 party the planar sphere should be really easy. Against skeleton lords bludgeon weapons are good (like against all skeletons). Try concentrate on one monster and after you kill it jump to the next one. That is a general idea, there are some exceptions but it generally works.

If you are on level 14 then you should go to Brynlow and Spellhold. Spellhold will be much harder if you enter there by a 15th level protagonist.

Okay, I will do that, thank you both