Make it simple though:
A kill = 10 points
Assists:
High Damage:8 points
Medium Damage: 5-6 points
Low Damage: 2 Points
No. That would increase the peasant killing.
You have the level of the target: T = Target
You have the level of the one hitting: A = Attacker
You have the damage dealt: D = Damage
The calculation will go faster just bringing all these values into one dynamic alghoritm.
Your own level will be taken into account as well, but not so much that it should be worth it being lower level.
Also, getting score just by being close to someone really needs to be minimal.
Do I need to remind you about the terrible old blob vs blob system...