Fusion is the process of merging your base card with catalyst cards. During this process your base monster will gain exp. Upon each level up your base monster will get increased stats in attack, defense, and hp. Each level on the base monster will take progressively more experience to gain the next level.
To fuse monsters a base monster must be selected. Selecting the base monster is done by clicking the change word near the top of the fusion screen, and then selecting a base. Catalyst monsters are selected by clicking the select box. A total of 10 catalyst monsters can be fused per fusion. The catalyst monsters are destroyed in the fusion process. The base monster will gain experience from each catalyst monster.
Each catalyst monster has a set amount of experience that it will give when fused to base monster. Normal monsters have the lowest experience with SR+ the highest. Also, levels on a catalyst monster will increase experience with higher levels giving more experience. During the fusing process catalyst monsters can have an experience bonus attached to them. If the class (Fire, Wind, or Water) matches there is increased exp. The same is true if the race is the same (Undead, Demon, Angel, Beast). The experience bonuses stack. Thus a Fire/Undead base will get more experience when fused with another Fire/Undead compared to being fused with a Water/Beast.
The experience cost for each level is progressive. A level 2 monster will take more experience to reach level 3 than a level 1 monster going to level 2.
Stat gains are not linear with level. A monster going from 39 to 40 will have greater stat gains than a monster going from 1 to 2.
At a level 1 base monster the fusion base cost is 200 gold. Each level up increases the base cost by 100 gold. A normal monster catalyst costs 200 gold, while a rare costs 400. Each level on a catalyst monster increases the cost by 100.
Exceeding Level Cap
Each monster has a level cap. For normal monsters the level cap is 20, while rare monsters are 40. When the level cap is exceeded any additional experience will be lost. There are no benefits or penalties to going over the level cap.