Warrior:
1-10: Stay at Maple Island and train on Green/Blue Snails then advance to Shrooms, Red Snails, and Orange Mushrooms.
10-18: Slime Tree at its best. If you don't like it, try heading for The Tree That Grew I.
18-23: Pig Beach is your best bet. Save some money for HP Potions!
23-30: Either stay at Pig Beach or move on to Green/Horned Mushroom tree dungeon at Ellinia, or if you're an advanced player advance to Ant Tunnel I.
Magician:
1-8: Maple Island. Always train on Green/Blue snails, and Shrooms.
8-13: Field West of Henesys, or the East.
13-18: Slime Tree.
18-22: Pig Beach.
22-30: Pig Beach, or Green/Horned Mushroom Tree Dungeon.
Bowman
1-10: Maple Island. Green/Blue/Red snails, Shrooms, Orange Mushrooms.
10-12: East/West Field of Henesys.
12-20: Slime Tree. This'll take some time.
20-25: Pig Beach. Will take some time.
25-30: Horned/Green Mushroom Tree Dungeon. Will take some time.
Thief (Dagger)
1-10: Maple Island. Green/Blue Snails, Shrooms.
10-19: Slime Tree. Will pwn with DS
19-25: Pig Beach. 2HKO or OHKO should deal with these pigs.
25-30: Horned/Green Mushroom Tree Dungeon.
Wishing to see some feedback, suggestion, etc.
Wishing not to see any flames. :-)