[Alneyan]

The problem with Shatter in this situation is that you need to spend at least one turn to get in melee (with Flying Boots), so you can cast the spell only four times at most, and Earth mages will definitively not default to Shatter. The 50% chance of losing a spell in melee may also affect the script of your mages, but I'm not sure.

Shatter also has harder prerequisites than Hand of Death (especially research), and only works on specific creatures, but deals the same amount of damage as Hand of Death, so you should use Hand of Death in these situations, if possible. There is, incidentally, no attack roll needed to hit with this sort of spell, so the Sphinx being immobile does not really help here.