The "chance to become invincible after a bird dies" perk is OP. I can't even deliberately let the birds die to end my game because something invariably becomes invincible after 3 or 4 go down. I'm typing this even as I let the game run. Maybe the probability to activate or the duration could be reduced?
The end bonus becomes insignificant almost immediately. 6 gold added onto the hundreds and eventual thousands that you get for just playing the level doesn't mean a whole lot, especially since it never goes up.
It would also help if they would double/triple team any enemies in range when they have nothing else to do. They do it at the very end when it's mostly pointless, but it does show that the armored enemies go down exponentially more quickly when they are being ganged up on.
Awesome game. Can I make a request for an Impossible Badge? S rank every stage, S rank every bonus boss, and get every medal. I don't know if it's quite hard enough for Impossible, but that's subjective of course.
For the boss speedruns, just summon as many shell shooters as you can. Don't even bother upgrading until you have a full set. Fly really close to the boss so the shots connect before they split for huge damage. If it doesn't touch damage you, then fly inside the boss.