In Kid Icarus: Uprising for the Nintendo 3DS, although the fusion system may seem random, it’s actually based on a fairly simple set of tables and numbers.
How to Fuse Specific Weapon Types
Certain types of weapons  fused together always make another specific type of weapon. For example, Bows plus Arms always make Blades, and Claws plus Blades always make Clubs.
Here is a complete chart of what weapon combinations make:
How to Make Specific Weapons
There is also a way to get a specific weapon.
There are twelve weapons of every weapon type, and every weapon in the game has a value added to it from 1 to 12. No two weapons in a category have the same value numbers, so the weapon type and number gives you a specific weapon.
Here is a chart showing all of the weapons and their values.
How this works is you take the value of the two fused weapons and add them together. If they are two numbers that equal a number between 2 to 12 then that’s your value. However if the result is greater than 12, then you subtract 12 from your number. For example, a weapon with a value of 11 fused with a weapon with a value of 8 would make a weapon with a value of 7 since your result of adding them together is 19 (19-12=7).
So using the above charts, lets try to make a Dark Pit Staff. What could we use?
We know that the Dark Pit Staff is a value 11 weapon, and that there are a lot of ways to get a Staff in the chart at the top, but we’ll just choose Arms plus Cannons. We need to get an 11, so we can fuse an Upperdash Arm with an EZ Cannon, or a Rail Cannon with a Drill Arm, or a Doom Cannon with an Electroshock Arm, and many more ways as well. As you can see, there are a lot of different ways to fuse each weapon.
Now keep in mind that there seems to be a bit of chaos in the listed table, and the game may spit out something seemingly random at times, so always try to save your game before you start fusing  a lot.
Fusing Weapons With Better Stats
Now, the tricky part is getting weapons with better stats. Just getting a certain kind of staff won’t necessarily make it very powerful, since the stats of the fused weapons is also important.
Basically, the better the stats are on the weapons you’re fusing, the better stats will be on the fused weapon.
Fusing weapons with similar stats will boost the stats slightly over time, so you can use this to fuse cheaper weapons and gradually get weapons with better stats.
