Addressing your northern forces is indeed in the list, and has the benefit of being one of the things that doesn't require me to spend gems moving my armies around. Addressing the lake and your southern forces are also on the list, but a tad bit more of an issue. I must say those shamblers have been vastly more effective than I thought they would be, although that likely can be attributed to their complete lack of opposition.
And yes, I certainly agree that the earthquakes were incredibly important in that battle-- as 20 gems of combat spells should be. I'm still questioning the decision to throw them on round 2 instead of round 1. Round 2 gave you an extra turn of buffing for your mages, I haven't yet checked how many of them cast vital life-saving buffs during the second round instead of the first. (All your surviving mages did cast some sort of life-saving buff though.). I could have cast them on round 1 of combat, but then I wouldn't have had summon earthpower, which would have meant a higher fatigue cost and less reinvigoration. (Because heaven forbid my mages not have 6 reinvig and 5(or 6 for that one guy) earth magic.) I also could have quad-casted it, but I was a tad worried about that potentially killing too much of my own stuff.
Anyways, that is likely to be the only time I cast Earthquake all game-- or at least the only time for a good long while. It /might/ make a reappearance later on if I start fielding darkness. (Since darkness lowers defense, which is what earthquake tests against) But it just isn't likely to be a useful spell against Lanka, Niefelheim, or R'yleh. And in almost any case, there are a lot of other things I can be doing with my earth gems.
Glad to see you still have a fighting spirit in you though.