Anything about Illithids is not off topic for a MA Oceania strategy thread.
Another thing I did fighting MA R'lyeh was to load up a couple Bane Lords with MR items, the works: Lead Shield, Ammy, Starshine Cap (okay, so I didn't bother with armor). Stick him at the front of your forces and watch as all the Illithids target him the entire fight and ignore the rest of your army. Unfortunately if you've got Sea Kings in your army then you need to use one of them as the decoy instead because they have higher HP.