Blue Mages, get ready to pound your keyboard in frustration. This spell for some reason will take you forever to learn. In fact, I still haven't learned it yet but I will celebrate the day I do get it.
For some strange reason, I thought that the respawn time on treants were set at 15 minutes. Because of this, I would often travel from one treant spawn to the next and make my way back to the first thus trying to get in 3 spawns before resting.
The respawn rate is around 5-6 minutes; much shorter than I thought it was. With this little piece of knowledge, I would suggest camping "Walking Tree" in Jugner Forest at either camps (J-10) or (F/G-7). It's well off the beaten path allowing you to concentrate on the treant with no wandering passersby trying to kill your prey.
~ The camp in (J-10) has Tigers, Orcs, Boggarts and Bogy's (night only). All of these aggro. Tigers and Orcs to sight, Boggarts to sounds and MP use, Bogy's to sound and low HP so becareful. All of these also aggro to resting to be prepared for some complications at the lower levels. The treant can spawn on both sides of the small river in that area and can spawn very close to the crag spine where the Boggarts are and up top on that small plateau. There are lots of treants in this area, moreso than I have seen in the other three camps so prepare for links.
~ The other camp is to the northwest at (F/G-7). The only thing you really will have to worry about are Goblins and Thunder Elementals during lightning weather. There are treants and tigers but not so much compared to the other locales. This might be a better option for low levels. The treant to the south in (F-9/10) is also not so bad but Orcs and Tigers are more plantiful here.
Like most guides say, get a lowe level weapon if your higher level and just feed it TP. I go BLU/WHM with Pro/Shell, Cocoon, and full MP before each fight. Dont cast Matallic Body or the mob wont gain TP. Do yourself a favor and don't even attempt this without capped BLU Magic. You will waste your time.
It really is as bad as everyone is saying.
Goodluck!