Only theorycrafting, but if I were boxing mages up against shammies, I'd go for either of these strategies.

1. Pop mirror image, water eles then invis and move to the centre of shammie grp - then go nuts with AoE - would work best with Belf mages since they could pop arcane torrent to prevent Shammies from using TS.
2. Pop mirror image, water eles, have an instant cast spell like Ice Lance on round robin to take care of groundings, use Iceblock to eat LvB and clear FS dots, cold snap immediately making Ice Block availble again.

Cheers,
S.