To keep your health up,you have to hide behind EVERYTHING that can help keep your life up.So remember crates are your friend and doors,if your by a door keep going back and forth and keep shooting until you WIN!Another thing that will help in open spaces is if you use Dash (over + B) that will help with bosses and just normal bad guys,it will still be helpful.So just remember that you just have to have lots of health just incase you have to face some bad guys or maybe one or two boss.