This guide is written based on my experience with IAH cbt + taiwan's OBT...
It is generally the same on where to level regardless of exp rate.

My leveling guide is based on a party of 4 and only grinding on Missions.

Things to note:
1. Always do Red colored quest (main storyline) as it is required for job change.
2. Try to always get a full party to do missions as number of mobs will be maximised.
3. This guide is for level 1-42
4. Add your party to your friendlist to earn buddy points which can be used to exchange for exp later

Level 1-6
1. Clear quest as you like.
2. Finish white wolf from F1-F5 and you should hit level 6/7

Level 6-12
1. Do the same as the white wolf clear F1-F5 and you should hit level 10 or 11.
2. Try to do F3/4 afew times together with the repeatable quest to get free pots and get to level 12.

Level 12-17
1. Do all quest on West Traiter map along with the hero quest.
2. Things to note: you have to do Konga's mission 1star twice
3. Finish up F1-F5 and grind on F4 till you are level 17(this is because toto's mission has alot of jumping to do and isn't really good for speed leveling).

Level 17-20
1. Finish up F1-F5 on Toto's mission and grind F4 afew times till you are 20 and do the change job quest.

Level 20-23(24)
1. Finish up F1-F5 on White Sharks's(Hookah) mission and level on F4 till level 23 or 24

Level 23-26
1. From lvl 23 onwards you can choose to go to the chaos map and level on Chaos Toto mission or proceed straight to Alvida if you feel you are strong enough or you have a good party
2. Do chaos Toto F4 till you are level 25

Level 24-28
1. Finish up F1-F5 on Alivida Mission and grind F4 till level 28

Level 28-32
1. Finish up F1-F5 on Atis mission and grind on F1-F4 till level 32
Note: You have to cycle through F1-F4 if you want collect the full level 34 set

Level 28
1. Finish up F1-5 for Dark Lord mission
Note: not worth grinding at dark lord as it is more of a "jumping quest" mission.

Level 35
1. Finish up Mole boss Mission
Note: not worth grinding on this, it has a few maze to tackle.

level 30-36
1. Grind on chaos Hookah F4

Level 36-38
1. Grind on chaos Alvida F1-F4

Level 36(38) - 48
1. Finish up Sambaba mission F1-F5
2. Grind F1-F2 for level 40 weapon
3. Grind F3-F4 for level 45 weapon
4. Grind F4 to level 42

level 43-48
1. Grind chaos Artis F4

Level 48-60
1. Grind chaos Sambaba F4