I did one of these caster build too, and had a slightly different experience. I found that the earthquake dc was great on a focused dc build, but then what? no real way to kill mobs so soloing was pretty much out of the question. So I moved some things around to up my dps, lowering my dc. And even though the dc was no longer no fail, it greatly reduced incoming damage. If it was 80% effective it meant out of 10 mobs, 8 would be on their back and 2 would be up fighting for about 2 seconds until they failed their next save. The mobs laying on the ground would block the others from attacking since mobs cant move through each other, and with beguile ticking away from ice strom and earthquake and the slower attack from mantle, meant I wasn't getting hit very much. Oh yeah, I have shroud displace clickies too.
But it meant I could kill mobs in ee all by myself, so no need for a party to bring dps for me.