If you can't wipe them outright turning it into a battle of attrition seems the way to go. We Shall Return costs 25e so they'll run dry eventually. Not sure how much energy they have though. I recall once hearing standard casters have 65e in HM, but I can't say that with any surety and Paragons obviously aren't casters. Perhaps try to organise the heroes to spike the enemies one at a time with Siege, hopefully prompting a Cavalier to burn 25e for next to nothing. I haven't vanquished the Desolation in an exceptionally long time (only did it once for the title) but I would think you'd burn them out eventually even without special strategies. Wurms have near unlimited rez potential too. Another thing to try would be to pull them so that they ball up and become more vulnerable to AoEs.
Also, if you're going the Frozen Soil route, don't underestimate spirit range. You should be able to cast it where it's not at risk of being attacked.
I very much doubt anyone left supporting GW would be confident enough with the system to go around messing with spawns, though it would be nice if they addressed certain enemies who have become disproportionately strong or weak due to updates.