Well it seems decided then. We could take advantage of the lich's skill by making stupid units. Keep it in reserve in case the enemy tries to make our people stupid. Or try to revise it into something else...
-Switch it from avoiding obvious things to more subtle things like shifts in battle lines or reacting to special threats. Probably resulting in a reduced power but consistent reduction of casualties?
-They could impart some of their mental faculties into their necromantic powers. Leaving the lich mentally tired(no stupidity protection) but granting the undead they summon increased intelligence?
-Focusing this power, along with curse magic, they might be able to impose their awareness onto one of the soldiers. The victim's head would probably explode after a while, but in the meantime their would have a front-line commander able to lead complex manoeuvres. It would probably be a bit like having a little miniature warrior champion. Except instead of being one big guy who can crush everything, it would be a squad of normal dudes with greatly enhanced coordination and positioning.
-A combination with curse magic to have them directly pit their mind against others, creating a stupidity beam that would make others as stupid as souless.
-Or we could make stupid units to work with the lich. They ought to get a price reduction for being stupid, but it is not our speciality, so it is unlikely to see the same efficiency as the soulless would have gotten on the smarts=>numbers exchange. At least for the first action. Also it is worth remembering that the lich is a champion, so we can expect them to only be in one plane at a time. If we really hate stupid, then this may be a poor choice. Although, bear in mind that at either end of the world there are less planes. If we rely upon our champion to buff our units then it gets easier at the ends and more difficult in the middle.
-Another option is to not build the lich. It is expensive, and it is no doubt a powerful unit. But mere knownledge of it grants us two magic disciplines, a coordination skill, and a template for powerful magic-users. If we just throw it away and instead focus on curse-wands, command imps, zombification spikes, and wizard ghosts, then we might not notice the difference...
-Your idea!
-just ignoring it is okay. Lich is still strong even without it.