I'm not sure how many of you would be playing this, but I would likely run this as a campaign type of game, with some actual role playing involved. Not just a simple skirmish game with no context.
Because of that, I'd handle XP a bit differently than how the games do it. Keeping tallies attack by attack for dozens of characters is just too much work. Instead, I'd just calculate the value of the skirmish that they faced, and distribute the XP to everyone equally (then modified by their current level).
BUT, I would also award bonus XP for cool shit happening, that the games couldn't really take into account.
-You just defeated that big boss in single combat? Awesome, +40 XP.
-You miraculously dodged those 5 attacks in a row, when you were 1 hit from death? Sweet, +15 XP.
-You put your own life at risk to rescue the wounded NPC? Nice, +20 XP.
On the flip side of things...
-You hid in the corner this entire battle and did nothing? Uhh, -50 XP
This way characters are still leveling up at different paces depending on how awesome they do stuff, and you don't have to do as much bookkeeping during the session. But if someone tries to game the system (ie put a low level character in the mission, but never put them in harms way), simply doc XP so they get none or at least not as much as the others.