User blog comment:Dudenorz/Brave Burst Levels?/@comment-107.217.200.227-20140203033636

Brave burst randomly levels up when you fuse the same type of unit.

There are three types of units; attacker, healer, and supporter. The elemental type has nothing to do with brave burst level ups.

The benefits of leveling up:

Attackers - the damage of the brave burst ability increases and increases the chance for abnormal state (depending on the BB).

Healer - the amount healed increases.

Supporter - the support ability parameters increase.

So if you have extra units that you're debating on selling go ahead and fuse it with the matching BB type and get it to max!

Tip: when you level up a character too high with metal gods, it will cost you more zel to level up BB so do it early.

Good luck!