The problem is, as with all non-100 range anti-SC battle spells (especially those costing gems) is that the enemy can easily script to stay out of your way until your script runs out, at which point the AI will try to cast something else entirely.
Is it confirmed that ethereality gives an undead 1 in 4 only to be subject by maggots? Would Luck also offer its usual 50% layer of protection on top of this?