Lost Odyssey

You will get this achievement when you gain his last level-based skill at level 49. To get his final skill (Ultimate Hit) you will have to complete the Royal Seals side quest, but it is not needed for this achievement.
Two good places to power-level are the Secret Cave (early in disk 4) and the Temple of Enlightenment (later in disk 4). At the temple he should gain a level ever fight until 49 easily. Just run around near the initial save point, fight the easiest monsters in that area, stick Tolten in the back and heal when necessary. By the time you reach that area, the initial monsters should be no problem for a properly-skilled immortals + 1 mortal to level up.

By the time you are able to access Temple Of Enlightenment, your immortals should be around level 50. What i did was, have all 4 immortals in the party, and have the 5th spot as one of the mortals. For Tolten, just keep him in your party till he is level 49. It should only take a few battles at Temple Of Enlightenment depending on what level he is starting at.

I found it much easier to, at the start of disk 4, with my party levels ranging from the high 20's (Cooke) to the upper 30s (Sed), go to the Secret Cave below Gongora's.

My party was Kaim, Sed, and Tolten in the front, and I rotated my mages and Mack in the back. You gain one level per battle against 5 of the Experiment 23 (or whatever number it is) until all characters are at level 48 or 49. Much easier and available sooner than the Temple of Enlightenment.