Suggestions:

Buff the birthday box drop rate. They are very rare compared to last year's drop rate.

Buff the elixir flask drop rate to what it was during last year's birthday event. 4x and 5x dmg elixirs are very useful for making a leaderboard push and there simply aren't enough.

Make kills count for all level toons.

Nerf waves 1-5 mobs/bosses (maybe even up to wave 10) so that lower level players are able to play the event. Capped players easily kill waves 1-5 so it won't make too much of a difference.