Ziggurat

This is awarded for simply reaching level 15 with your character, difficulty doesn`t matter. You can reach that level with every character and probably will if you clear every room on the map, but the easiest one is Cid the Seer.
He is unlocked after killing 250 carrots and does gain xp faster than the other chars. Take note that has quite low HP in the beginning, so take care.
There are a few perks to help with getting xp faster, but those have to be unlocked first:
1. more xp from xp-crystals
2. xp from jewelry

Collect the scrolls from the scroll rooms for a nice xp-boost and pick up the xp-items quite fast, since they dissapear after a while when not collected.