I personally find training at Path to Caballa Relics a decent place to train from about 35 to...dunno, 50? Let's look at the essentials.
Good Spawn density. While it's not the most monster-dense place in the world, if you're willing to kill both Leaf Birds and Mandragora, there's plenty of monsters to go around (I can kill ~25 Leaf Birds in 3.5 minutes when 3HKOing with Burning Rave). More a place for those with higher durability/damage to train due to aggro Leafbirds.
Close to repot. The nearest potions shop is one map away in Gate to Caballa Relics, stocking potions in the B and C ranks. Mandragora and Leafbirds possess no spells (though I believe they might have Soil element due to animation...not much, though) and thus are not a major threat (no banish, no mana spells, no elemental spells, nothing). By comparison, the nearest repot for Phantom School 1F is...what, at least three maps away? I don't know where it is, all I know is it's off the second floor annex...which is where?
Size. While it's not the BIGGEST map in the world, it's probably capable of holding a party of six. It's not PUNY, per se, but it's small enough to allow for a decent respawn.
Loot. While there's not that much that's extremely valuable, you can occasionally get LV50 stat stones. The normal drops are enough to cover the repot and then some (typically).
AFK location. Portal to the Southwest Forest...around there is all Mandragora if anything at all. No definite AFK zone, but it's reasonable.
Also, random training tip: Set yourself little "monster quests". I personally enjoy racing against music to see how many kills I can get before the song ends. My song of choice is probably the SSH remix of Tales of Phantasia's "Fighting of the Spirit". I try and see how many kills I can get in the 3.5-minute song, and training feels faster because I'm less "I'm bored, training is going slow" and more "Aw crap, the song's about to end!". It goes without saying to use your own music when training (if you prefer it over the BGM), but I haven't seen it suggested to use the music as some sort of timer/race/motivation. Thought I'd mention something that helps me out.
Sorry if I'm posting in a thread that has its last post four months ago, but...