Good system perhaps, but too little information to player. Game should inform player of all important details, like XP.
As for thread, I say each use of the skill should give XP. Failure in that skill rewards let's say half of the XP, where as successful use rewards full XP to the character.
Battles should have 3 XP pools: Base, Damage and Killing Shot.
Base XP is something that is given to everyone.
Damage XP is given based on how much damage each one did.
Killing Shot is XP boost given to one who gives killing blow.
This way it even out, one who makes that 99% of damage get 99% of Damage XP and one who kills it gets 1% Damage XP, but Killing Shot boost. Everyone gets Base XP, which can represent pretty much anything, like "look and learn"- So if one guy does all the work, he gets highest XP. On the other hand, others get token XP too. If all participated in killing the target, they all get slightly more XP.
How does that sound?
