...I agree, I've noticed: the bigger the better/faster. I found myself leveling up 'faster' with the Staff of Embers than the Ice Staff[which I made first and used first]. I think this works for all weapons and XP gaining; the more damage it delivers[not enemies current HP total], the faster the Skill upgrades, or so it seems to this gamer. So, I use the weapons in each class with the most damage delivered to upgrade that Skill. I'm currently using the Carapace Spear(I NEVER used the spear before, lowest Skill on board) and it only takes a few hits, in an early biome to upgrade the Skill....and this weapon seems to deliver the most damage for type of all the weapons. It seems to be upgrading incredibly fast...

You 'might' think that the XP would be based upon current NPC health level and not a broad sweeping, 'one size fits all' approach...pleeze don't!