magic guard should only be used in areas where mobs can 1hko you. sometimes you might want to use if if they 2hko, but thats already a waste.
you can also use magic guard IF AND ONLY IF you dont actually even have to use MP pots due to mp eater and mp regen.
if you are a cleric, dont use magic guard unless it is 1hko.
priests with 1k hp are known to fight at grims without magic guard. grims do about 900+ damage! why? cuz heal can hurt them. so just keep healing.