First thing's first, welcome to Mazey Flyff!
I recommend Arcanist if you want to farm RC/RP and Mazey Quest Chips (MQCs)
If you want to run dungeons and slay bosses, there are 3 classes to choose from:
1. Slayer
Slayer holds the highest PvE damage here and majority of players here also uses this class to hunt bosses.
2. Seraph.
Seraph--though I could be wrong here--probably holds the 2nd place for PvE damage (just below Slayer). I'm always a fan of this class. The reason being, you can always buff yourself when you die. Especially when you are doing Colosseum where there are times that you will die.
3. Crackshooter.
Crackshooter are often times used in hunting Red Meteo. Red Meteo has the highest selling value drop of all the dungeon bosses. It drops MasterKing Shiffu Shield which cost around 90 RP+. And Rarity Upgrade PvP Shield if you are lucky. The main reason is because of its range which is very useful in landing the first hit on the boss.
Here are some guides regarding the classes that I mentioned:
Slayer GuideSeraph GuideCrackshooter Guide.
And for Arcanist, you really don't need fancy stuff. For non-donor, you just need a Kalgas Arcanist Set; Catalyst/Black/Shadow Staff; Legion/Holy Shield; Attack Set Jewelry; Cloak of Honor; Pick-up Pet with +3 lucky drop awake.
Though you don't have to follow all the gears I listed. I mean the important things you should have if you will farm are Cloak of Honor which gives +1 lucky box and Pick-up Pet with +3 lucky box awake. Then just use any random staff. But if you want to maximize your damage for fast farming, then follow the gears I listed.
Good luck!
P.S. You can always PM any member of the staff if you have more questions. They are always around. Just type %gmlist in-game. And you can always PM me at Shai but I'm rarely online in-game though.