Diamond Hollow II/YMMV


 * Game Breaker: The special abilities for the rebound gun, triple ricochet and the ability to ricochet off enemies. Get them and turn on autofire, then watch the screen flood with projectiles until everything in your path is destroyed.
 * That One Boss: The snail king.
 * The stone bird can be pretty difficult too, though it's more of an Early Bird Boss (...) since you only have two (low-level) guns at that point, neither of which are really optimal for dealing with it.