I've attempted this battlefield a few times now, the lowest I got the Bygone Geomancer down to is 47%. I have a chart and am only nuking opposing elements as suggested. I'm running around to spread out the luopans. I keep indi-regen up for the geomancers magic spells and attempt to use a Geo-refresh when I can, utilizing Mending Halation/Radial Arcana when it is up. Anyone have any further advice? 15 minutes just seems like such a short time for this BC.
This thread might also serve as a good place to put strategies that have worked.