So, fusion is pretty straight forward in Marvel: War of Heroes. Fusion itself is the process of combining two of the same card to increase its potential power as well as its rarity.
A card that is uncommon quality fused with another uncommon card will be boosted up to rare quality. Each card will also have 5% of its current stat points carried over to the final version if it is not maxed. If the card is max level it will carry over 10% of its current stat points.
So, to show how this works I will use cards that don't exist with stats that don't exist but will illustrate the point:
Emma Frost: 1000 base attack 1000 base defense
3000 max attack 3000 max defense
Emma Frost+: 2000 base attack 2000 base defense
4000 base attack 4000 base defense
1) Using this example, if two base Emma Frost are combined they will each carry over 5% of 1000 atk/def to the final form, so 50pts each. This means the final form will begin at 2100atk/def and end at 4100atk/def.
2) Using the same example, if two max Emma Frost are combined they will each carry over 10% of 3000atk/def to the final form, so 300pts each. This means the final form will begin at 2600atk/def and end at 4600atk/def. Obviously this is a large variance.
Now, Fusion does not end with pure levels, there is also Mastery to be considered. Mastery increases a cards stats by roughly 37% of the base stats once it is maxed. If it is not maxed it doesn't increase the cards stats.
3) Using the same example from above, an Emma Frost maxes level and mastery resulting in 3370atk/def. Using two max level/mastery Emma Frost she will retain 10% of 3370atk/def in the final form, so 337pts each. This means the final form will begin at 2674afk/def and end at 4674atk/def. Obviously this variance seems small.
Now, at first glance it appears that leveling mastery isn't very important, but it can be if the cards base stats are high. There is also a perceived value to people you want to trade with eventually. The easiest way to computer the mastery stats is the end result will be an extra 7.4% of the base stat of the card if you max mastery and level on both.
Also, if a card is Fused at max mastery, it will carry over 100% of its mastery points to the final form, saving you on work. Because of this I always view mastery as free points and should be maxed as much as possible.
Hopefully this helps with any questions on fusion.