Everything hinges on where you are in the game.

1) The following GF's know HP-J, allowing you to junction HP to spells:
Quezecoatl
Ifrit
Brothers
Diablos
Carbuncle


2) The following GF's know HP+80% that raises your character's HP by 80% when equipped:
Brothers
Diablos

To get a third GF with HP+80%, you need to teach it with a Gaea's Ring.

3) The following GF's know STR-J:
Shiva
Ifrit
Pandemonia
Cerberus

4) Use Hyper Wrists to teach your GFs Str+60%.

5) Ultima is the best way to go when junctioning HP to spells. You get a whopping 6000 boost with 100 Ultimas. If you don't have Ultima yet 100 Full-Lives will give you a 4800 boost. Also Meteor, Flare, and Aura give good boosts. Failing that use Tornado, Triple, Pain, or Curega.