A "weapon that levels up" is likely something you won't be able to do without scripting, so I suggest you start to familiarize yourself with the RGSS system and Ruby if you haven't already.
If you want to lock an item to the actor without writing a script, you could do so by having classes specific to an actor, and then checking off items and skills specific to those actors within the classes. For example, if you wanted two actors to both be "Lancers", you could, in essence, create two "Lancer" classes, each for the two different actors, so that they could each have unique items/abilities but appear to be the same class.
As for the dynamic weapons, I can't really see any other way of doing this besides a script. You would need to find a way to check what level the actor was (perhaps on level-up at the end of battle), and then replace the weapon with a different ID for each tier (I think modifying the stats for a single weapon individually is more complicated, and this would allow you more flexibility than just a single stat).
There are actually two variables built into RPG Maker XP that handle this, and they're both in the "Game_Actor" class; actor.level and actor.weapon_id.
So, you could create some sort of method to check this when the actor levels up. Something along the lines of:
Code:
def get_weapon_tier(actor)
case actor.level
when 1..9
return 1
when 10..19
return 2
when 20..19
return 3
...etc
When used, this method would check what level the actor was, then return the weapon tier. Then, you could use that value to set the weapon to something different for each tier. This is obviously not plug-and-play code, but I think learning how to do it yourself is going to help you out the most. Hopefully a hint on the logic helps
