Well, if Grade 12 isn't doing it for you, just buff it to Grade 13. Or make a dual or triple gem -- those get damage bonuses (duals get +15%, triples get +22% if you max the corresponding skill).
If you have spare skill points, you can put them in Resonance to buff your damage output.
Or you can Take A Third Option: build up to about 20k mana, then take a high-grade gem (at least G9 should do it) and start combining it with sheets of Grade 1 gems. You'll waste your gem's special abilities, but its max damage will rise (albeit very slowly). There should be no problem reaching 1000 max damage this way.