Leeching method for my characters

4 at once!!! just put everything on your party and be happy

 

Leech all four of them in a party and here's a list of leeching spots taken from here.

I took some tips and advice leveling up with mules from this guide.
Levels 10-21 can be done in a few ways optimally. However, I did Henesys PQ which gave maybe nice exp for minimal effort while watching a show/movie on the side. Collect the seeds and just lure away using warrior with slash blast or use your priest/bishop to lure and kill.
As for HS and EXP formulas related, here's something by Nightz777

 

For sure. Leeching 4 at a time is going to generate the most possible XP per minute. Per-character it's going to feel VERY slow, but mathematically the most efficient.

Some tips:

Have max Holy Symbol (obviously)
Get Genesis at level 120, but only keep a single point in it to reduce mana consumption and thus overall pot expenses.
Mystic Door is going to help you move all the character around. One of the most annoying parts will be getting all your characters into place.
From levels 10-20, doing Henesys PQ will be very efficient.
Review the list of quests. Certain quests, such as the quests where you have to kill 100 monsters or kill 1 boss are quite efficient at scale!
Finally, think about possible ways to do the party quests. If you can find or get a PQ mule to carry your characters through LPQ or EPQ it will be a nice break from leeching!

Good luck! I've just been leeching 2 character with my bishop, both level 53 now... And it's quite more time and effort than I was expecting!

 

Another tip about quest, is collect/buy etcs from high exp quest, Farm with a bs, or make a owl mule and buy it, like strange dish eye tail, Phantom seeds