Find a location that only as a portal that lets out birds with nothing else around to interrupt you. As soon as the birds appear, kill them with you arm swords, but make sure that you do not hit the portal or it will be destroyed after a few hits. The left arm levels up more quickly than the right arm. However, with some time you can get them leveled to a decent level at the start of the game to help you in the Boss fights that happen later.