Make sure you are doing all the side quest AKA (BLUE) ones!
I always recommend people saving there cards.
As this game has brackets for leveling!
http://www.tosbase.com/game/exp-tables/base/
you will notice as you level you will be required to get more exp. and at certain levels like 86 and 136 the following levels require a lot less.
So i suggest leveling at royal mausoleum! FYI there are two of them the main story line one and the dungeon for grinding mobs.
Once you have finished all the quest gold and blue ones for your level! Also keep in mind not all quest are handed to you. You will have to venture and look for them. in game press “N” for world map and go check out maps your level to get more side quest.
Ater all quest are done and you have racked up a whole bunch of cards. start grinding.
Royal Mausoleum will keep you busy all the way to 80-86. Once you feel the grind is unbearable for you. SPAM your cards to skip the the next bracket. which in my example would be 86 wich is ruffly 468K EXP and 87 only required 20K EXP.
And just repeat this process according to your level range and dungeons. 100+ i believe the dungeon is called “Demon prison”. This should keep you busy for a while. HAHA
Hope this helps, Sorry for my broken English.