Super short ideas:

Keep the Rainbow going until the Leprechaun is defeated
Why? So people have an idea which way to go to get started at least

Delay the spawn by... like 30 seconds?
Why? So more people have the chance to join in and for the Leprechaun to scale to the group.

Give the Leprechaun a Glowing Effect
Why? Cause people have difficulty finding the Leprechaun at spawn after it has spawned.

Together, it should make it a lot easier for people to participate.

Difficulty: 
I can't imagine any of these changes is much more than a bit of tweaks.

Nice ideas here Bowman! 

I could see how glowing helps, perhaps he glows just for a few seconds at first.