One of the reasons that people have trouble training Bagis in 2Moons is because of the limited skills at lower level. However, it's quite easy to level a Bagi if you know how to do so.
1. From level 1-11, train on the outskirts of Loa or Braiken castle. Training here should be a breeze.
2. Once you've achieved level 11, enter the Maze of Newbies. This is a excellent solo dungeon that gives you both potions and experience at the same time.
3. Once you've achieved level 19, you have two choices to train at: at denebe or inside norak/castor cave, 1st floor. Both are equally adequate, but keep in mind denebe is a non-pvp map, and norak/castor has pvp enabled, meaning anyone can player-kill you while your inside the cave.
4. At level 25 (or sooner, however you prefer), head to Heiharp and see how you fare there. The best experience you can get is with a party, mainly because it takes less time and less work to kill a group of monsters with a party. It's up to you to train here until you've reached level 34, or you can try Requies, a jungle like map just north of denebe.
5. Once you reach 34, head into the Requies dungeon. This is a solo dungeon that requires you to do several tasks to complete the entire dungeon. Upon completion, you earn an additional 50,000 experience points. You may not be able to complete this dungeon the couple first tries or even until your late 30's, but this dungeon is by far, the best experience you can get at this stage. Once you've reached the low 40's, this dungeon should be much easier. There's also an optional boss you can attempt to kill by getting the 3 relics hidden inside the Chamber of Space. Killing this boss rewards you with a very nice ring. I suggest you do this dungeon until 55, since it's still the best experience you can get.
6. Once you've reached 55 (or 50), you can do one of three things: train inside of norak/castor cave by yourself or with a party, train inside Parca Shrine, or head out to crespo to train there. Parca Shrine isn't the best area for experience points, mainly just for commission points. At level 55, crespo may be a bit difficult, unless you find a party to do the commission quest and train as well.
7. During your training from 55-60, you should save up on money. Upon reaching 60, you can now enter the Dungeon of Crespo, a party dungeon that has three section (Dungeon C, Dungeon B, Dungeon A) and costs 300k to enter. However, if you go with a party of 6, each party member only needs to pay 50k. This is a quite decent dungeon that gives you experience and different gems that you may find useful, such as medials, rhombuses, elemental damage gems, or elemental resistance gems. You can also continue training in crespo doing the commission quest (s) with a party or just soloing, however you enjoy it.
8. Once you've reached 65, you can have three options to choose from: continue training in Dungeon C, head out to Draco Desert, or do Dungeon B, which is harder than Dungeon C, because of the higher level mobs. However, it does give more experience and better gem rewards. Draco desert is a different view from crespo, mainly because it's a desert, unlike the snow you constantly see in crespo. I wouldn't advise doing the 65 commission quest, because it's just a waste of money. I suggest doing dungeon B from 65+.
9. Once you've reached 70 (Hurray!), continue training in Dungeon B or head out to Draco desert where it's a bit easier to solo train here while also doing the level 70 commission for money and adventure points.
10. Upon reach the big 75 (halfway there), you can attempt doing Dungeon A, which is the hardest of the three sections, but also gives better experience and gems. However, if you find it a bit hard, continue doing dungeon B.
11. Once you've reached level 78, you begin to slow down on experience if you continue doing B. This is when I would suggest for you to go either Dungeon A, or Draco desert for the commission quests while gaining experience. However, the Dungeon gives quicker experience for a price.
12. Once you've made 80(2Moons powerleveling, 2Moons gold), you can either continue in Dungeon A or head out to Python Castle. I highly suggest you to get with a group party in Python, because it's very hard to solo there. The experience here is decent, since it gives the same as the Dungeon B experience (in a full party) and you can also make money here from the money lost doing all the dungeons.