You must max one class of Marumaro's Sabre-Tooth Shadow to level 99, it does not matter which class it is as long as you level it to 99. Now for some pointers:
-DO NOT use the hearts for that specific class to go from 98-99 this will forfeit the achievement so don't risk it. After Level 60 the SP needed to level up will remain at 100, this is the time to use the hearts.
-Find as many double SP accessories as possible, here are there locations:
1. Black Belt - If you find 1200 nothings, return to the nothing man for this reward.
2. Black Belt - From a chest in the Ancient Ruins Forest
3. Black Belt - From a chest in Mural Village, it is covered by a Red Barrier.
4. Black Belt - Steal one from Yeet Yeet monster (only works on the first steal attempt.)
5. Eye Patch - You will find a Autograph Manuscript in the desert region (where the crystal "sword" things come up through the ground), trade this to the traveler on disk 3 in the Lal Mountain Camp.
Now here is the quickest procedure:
First Level up the Barrier Class for someone up to level 50, when you get to level 50 you will receive the Field Barrier 3 ability. This field ability lets you just run through enemies and collect their SP, but no gold, items, or exp.
Hands down the fastest place to level up your Shadows is in the Laser Field, warp to the Laser Field and head North, you will see a chest in a little alcove where the purple flying mole enemies will endlessly respawn directly in front of the chest. Just activate Field Barrier 3 and stand in front of the chest, the enemies will spawn right on your barrier.
Now for each enemy the field kills it will consume 1 MP, you can do one of two things; either equip an MP Regen ability, (but you will have to be moving) or you can simply check back in on your game every once in a while and restore your MP manually.