You should level 3-4x as fast on Retz as you would on Lorangs. Around level(MapleStory gold) 55, you could also consider drakes meal table as a decent training spot. The exp is in general better then Retz, you will gain 85 + sometimes a 240 exp times 6 a few seconds or 135 times 6 a few seconds.
The drops are also noteworthy, as wild cargo's sometimes drop tobi's, and Drakes quite often kumbi's. The only big drawback of this place is that it drains meso's. Especially with magic guard on, which is a must since cargo's do around 300 damage in Maplestory. Expect to earn half of what you need for the potions.
For lightning wizards, try going into the room with the roids and neohuroids in Magatia, known as C-2. It has a lot of monsters, and you can mob them to the edge of the map where there is a safe spot to zap them with lightning. We provide MapleStory powerleveling for your character.