Fusion

Using the fuser NPC, monsters of the same rarity can be fused together to increase the monster's maximum level and give them a boost in experience.
Monsters in Tarochi have a maximum level of 10, but every fusion increases the maximum by 1 up to lvl 20
Fusion also gives experience points according to (1 / monsterLevel). Note that the formula is only dependent on the level of the stronger monster in the merge, and isn't affected by Genesis Trainer ownership