-Note: This is very risky but you can make about 20,000 Mesos in a hour. Make your way to the Ant Hill through the dungeon (going there from Henesy recommended). Then, once you are in Sleepywood, go east and there will be a portal. If you go in there and find a person not taking the money, take it from them. Note: This is very dangerous because there are high level Horny Mushrooms and Zombie Mushrooms.
-If you are level 35 or higher, hunt Sakura Cellions on Amoria: Purple Plains, which give about 232 to 348 Mesos per kill. They are level 33, have 1100 HP, and drop Cellion Tails, which are exchange quest items and sell for quite a bit. They also drop level 50 equips such as the Thief Red China Set (male), level 50 warrior helm that can be sold for quite a bit, if its over average on STR; and the Red Anakamoon Level 58 magician overall and cape for STR 100%. They do not do much damage. You can get more than 300,000 Mesos an hour by doing this. Note: Many people go there and might "kill steal" you.
-Make a new account in the same world as the person that you want to make rich. Get the new character to a good training ground. Kill things that drop good items and collect them. Go to the bank. Store all the expensive items at the bank. When your bank is full, take everything out and sell them.