One your stamina is ~350/400 you don't need to bother training it much more, concentrate on reactions and agility then speed and keep an eye on your health
It's probably a glitch somewhere, but I reset data after completed the game and when I started again, my prime gun was firing a three-spread of bullets
the space-bar select command is really useful for completely out gunning your opponents, just keep sending clouds and you will overpower them regardless of their upgrades
Anything is easy if you practice enough. ;)