WEPONS GAIN XP EVRY BAT TLE

You mean fusion right?

I think that the suggestion is that every time you battle your weapons get more exp so an alternative way to level up your parts other than through fusing.

I see, I don't know tho...
• What happens to the XP when I fuse said item to max?
• Can I use my general XP to upgrade said item, or visa versa?

The way you can math it out is easy for weapons to gain XP(Fusion) during each battle. For instance lets say for every weapon you use on an attack the amount of "HIT POINTS" you damage on the opponent that's what you get on that weapon. So lets say the 1st attack was 36 "Hit Points" releasing a drone, that be 36 XP(Fusion) for the Drone. That be the best way to do it, I think.