How do you power level on Maple Story

Due to the stupid level restrictions, a person can only gain exp in a party if the person is 5 level's within the monsters level range.
A slime, for example, is level 10. In order for a person to get exp, they need to be levels 5-15.
You could try to buy lots of potions. Then you go out monsters and fight but don't pick up the money unless there are some items because it takes time to pick up everything.
EDIT: You can leech, so say if you lvl 75 you can buy hak leech. This may be patched but you can get PG leeched where 1 person has a HP warrior around the same level as you and damages any lvl monster and when they kill, you get full amount of shared exp.