You mean the exact same thing every other mage jobs has to do? Except to a lesser degree because of a lower skill cap to reach in the first place?
You have to cast on things appropriate to your skill level, i.e. things you will have absolutely no trouble dealing with, because you have enough skill to deal with them+ level correction and higher stats on your side.
Or learn to play the game. Go to abyssea ulegeurand range and ignore everything I just said. Turn off arts, eat pitarus, cast spell on worms that can't hit you. Problem solved, you can stop complaining how terrible you are.
I'd just like to see GEO get healing magic skill natively. You have to stand next to the enemies anyway and you sure as hell aren't contributing anything useful with nukes or melee while you stand around. I'd like to say give them the cura spells and let them keep the melees alive while they keep debuffs up, heck maybe even let curas hit the Luopans as well. But considering how Cura spells were wasted and given to WHM, and only having them work when under afflatus misery, there'd need to be work-arounds there. I guess normal cures would work too.