Once you just start play Pet Forest. There have drop a Lucky Packet (at least 10 pcs for lv8, 20 pcs for lv10, 30 pcs for lv 12) near the disciple and tell him/her to use it from your leveling up. Then you can go to Chaotic Oasis in Magical Ocean and kill Lv22 Sand Bandits (1339, 3300) in the lower part of the Haven for fast mob killing or go to Lv20 Smugglers (1624, 3017) in Ascaron which is also the same.

After you have done. You need to go to Magical Ocean and kill Lv30 Lizard King (1507, 2970) which is also fast mob killing. You can either go to Abandon Mine 1 (1883, 2805) and kill mobs but in smaller amount so this is the slowest part which takes at least hours to level or on Abandon Mine 2 with Lv38 Ninja Moles which is also slower and lower quantity. Although, this is much safer place than other spots to level disciples.

Then you can level on Deep Blue near Skeleton Haven and kill Lv38 Ferocious Scorpion (2373, 551) which has more mobs than in Abandon Mine or Lv39 Snowy Task Boar (2266, 590) w/ Lv 36 Crazy Sheep (2266, 590) which has smaller mobs than Scorpion but more than in Abandon Mine. You can level at Abandon Mine 3 in Lv 46 Pumpkin Knights which has 5 mobs per respawn place and it has safe spot near the entrance.

Once you are at level 37, you can level(Pet Forest Gold) at Ascaron near Chaldea Haven on Lv 48 Treant (828, 1923) which is faster and higher rate of mobs but far more dangerous because of many mobs respawning and the disciple must follow the mentor closely to survive.