I understand that it's frustrating (well, not for the leaders / casters, who are drinking from the XP firehose). I know it doesn't completely fix the problem, but the next two scenarios have features that I think will mitigate the differences somewhat.
In one of these scenarios, the current situation is reversed. You defend the city. I'll be adding 2 heavy ballistas (stationary, requires Heavy to fire, grants Fire attack for 3d6 + Combat) so the heavies can join in the shooting. There will be significantly more gump-level units, so the leadership bonus, while still valid, will be granting XP on a much lower ratio as higher levels progress. The other scenario will be a more traditional dungeon-crawl type in an enclosed space, so range will not really be a factor at all.
Also keep in mind that, in the current scenario, if it weren't for the Bats, all of your fliers would currently be dead (getting shot down as they entered the field), there would be twice as many archers (since the fliers dropped most of them), and the bodyguards and casters would be the next targets. I've gamed this scenario in the players' favor significantly, because I wanted to see how things behave past the first few levels, but as has been pointed out in the past, Healers / Leaders / Casters are usually the primary targets. They'll level faster because (in theory) over time, they die faster. Five rounds isn't enough to demonstrate this.
I'll confess that I'm not really interested in running scenarios when two sides just line up against each other and smash until one side wins. I want to see strategy, forethought, and complex tactics, not just number-crunching. I want to be fair to all players as well, and I want everyone to have a good time. I also want situations where players level in the middle of a battle, and a new ability turns the tide.
After this scenario completes, we'll do a post-mortem on what you all feel works and doesn't work.