CryMore,Better bows are obtained by your total amount of points from all the levels,once it hits a certain amount(it even says in the shop)It will be unlocked for purchase.And yes,the "3 grade points required" or w/e means you need 3 MORE gradepoints to unlock the level.My advice is just keep trying,and keep enemies behind blue line,and don't miss.
Lvl 100 you get your first star,then 120 for 2nd,130 for 3rd,and so on until you get your 6th.When you get your sixth star,you never reset level again.
...You've got to be kidding me,reading all the "2ND BOSS IS IMPOSSIBLE!!!!" comments,i expected him to be hard.He was EXTREMELY EASY, he died in about 5 seconds to me,i was never hit :/.What in gods name is so difficult about him?
 "Hunter Story")
 "Run Elephant Run")
 "Run And Gun")
 "Stick War")
 "Math Challenge")
 "A Knight's Story")
 "Medieval Rampage 2")
 "Bounce")
 "Frantic 2")
 "Helios: The Defender")
 "Alpha Bounty")
 "Enigmata")
 "Castle Quest")