My highest score was 4,213 in Nightmare difficulty! I happened upon a nice seed of upgrades and retried until I could sort them how I wanted. The MVP Black had Mimic and was copying Infini-Blast for most of the game. I sat Black in the middle and recharged his ammo with Blue. Pink was damage control for the edges and Blue allowed Pink to access the opposite sides with Lane Wrapper. It was super satisfying. 
After getting a score I felt I couldn't beat unless I choose my own upgrades, I invented my own challenges of beating Survival and Onslaught with only 2 gunners (the third must hang back and NEVER shoot, but can still pick up upgrades and even Share Skills or Ammo Sharer). It was a fun goal, I even managed to beat Survival with a single gunner! Although that took quite a few tries...
I'm aware there are space limitations, but all the fun I had made me yearn for an unlockable sandbox mode where I can choose my own upgrades. The game doesn't need it, though!
All around an excellent game! I'm collecting a few of my favorite Pico-8 games to put on a homemade device, and this will definitely be one of them (Buns: Bunny Survivor, Solitomb, and From Rust to Ash are a few others)!

For those who are curious, it simply changes to "Coming Soon" once you click on it.