Maybe you could track the amount of xp ever gained instead of the amount you currently have.
(Like the “XP Gained” group on the Statistics page) (If that’s what that does)
Then just raise the amount much higher so it’s nearly just as hard to get but isn’t as frustrating for players when they die and lose ALL their levels.
This would stop players from being able to just get free levels from a friend to give right back after ranking up. (They have to kill the monsters themselves) As long as buying xp from signs doesn’t add to the xp gained.
(I’m assuming you’ll think of some reason not to do this, though)