Use spiritsong (halves MP costs for 20secs) and do level 20 guildleves until level 24. You get fire II, Aero II, Blizzard II and so forth and you'll start doing dmg again.

If you want easy mobs to solo go to 38, 23 in La Norscea - just north of Gullperch Tower. Crabs there are level 20 - 22 and gives from 100 - 175exp each. You can easily take down a group of 3 - 4 without breaking a sweat.

Leveling THM to level 20 / 24 also helps with the mana.