Another random bit of weirdness you might want to try:
Darkness.
It's not exactly an easy spell for R'lyeh to cast based on its national mages, but if you can get there (with communion, this might be not that hard late-game), but it reduces the precision, attack and defence of every living thing on the battlefield. This matters a little less for illithids than for others, because mindbLasting always hits even if they're *blind* (so Solar Brilliance is also more of an option for R'lyeh than most, incidentally.) R'lyeh is also penalized less by Storms than many, due to lack of fliers or archers and again the prec-100 mindbLasts.
(Storms + Darkness + Stellar Brilliance would be overkill, but might be worth trying once.
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