EXP Gerico

Many Players considered EXP Gerico to be the most useful card to give a considerable amount of experience. However in the longer run, it gets quite expensive as your hero level increases. 

To make this easier for us. Here's a few tips that you can use whenever you feel like leveling your heroes a little bit.

1. Make sure you have enough Gold and Courage Gem to buy mass quantity of  EXP Gerico. Some high hero levels requires more than 50 Gerico EXP to level up.
2. Obtaining Gold is easy; always do the limit gold cap for event boss. 50m is a good quantity for leveling heroes; you can also buy treasure chest(i usually save up to 50k chests) that way you can generate a decent amount of gold; 10m per day is also helpful if your guild level is 100 and you are the guild leader.
3. If you're not buying Hero Pieces from courage market, just stack some Gerico Pieces in your warehouse.
4. The game is base on luck and algorithm; luck, meaning you have a chance to get the fortification bonuses for a massive quantity of Gericos; algorithm, meaning fortify 1 Gerico Piece to further increase your fortification chances.
5. There are some Heroes like OPHELIA and MAI SHIRANUI that can be leveled easily; use them as your experience grinder and just transfer them to your other units. Its a waste of resources but good for level grinding.

There are other players in each server that knows pretty well how to level up their heroes. This guide is just to give you some ideas. You can always ask the veterans for help.