On the island you obtain the airship, there are some enemies that have the Death Sickle, which can be stolen and given to Berserkers to do crap ton of damage as well as random instant death.

With this, you can go to a cave near the lost city where the Earth Crystal resides and farm Metal Squirrels for AP. Otherwis its damn near impossible to kill those enemies with your current levels and job classes. One of the better places for AP farming in the early parts of the game.