Good Place to Level Up *minor spoilers*

[Xurts]

Here's what I do:
Have Quetzacoatl learn Card and the card refining ability. Have Diablos learn the time/space magic refining ability, and Siren learn the Life/Recovery magic refining ability. Refine Diablos's card into 100 Black Holes, and refine those into Demi's for all of your party members. Refine Sacred's (or maybe it was Minotaur's) card into 100 Dino Bones and refine those into quakes. Junction those to your HP and Strength.

Sacred's card mods into 100 Dino Bones, Minotaur's into 10 Adamantine.

Quake is great for strength early on, but Tornado is stronger once it's available. I wouldn't recommend Demi for HP, Regen and Full-Life are a much better choice.

Go to Cetra and get Odin, you might want Diablos to learn Enc-None so you have plenty of time to fight him. He won't do anything at all, but you have a time limit, and if it runs out in battle, it's game over. Refine his card into 100...somethings...that refine into Deaths. Junction those wherever you want.

Speaking of Odin, when you fight him make sure to draw 300 Triples. Odin is by far the best source for Triples, so if you fight him then definitely draw as much as you possibly can (300!). Odin's card refines into 100 Dead Spirits, which refine further into Deaths. The best place for that is either Status Defense or Attack. I put Death on 1 character's Status Attack, that way it's a 1 hit kill if the enemy isn't immune to it.