In order to reflect an enemies projectiles, you will need a shield equipped. During battles you will see quite a few green/red icons above your characters head. 3 green lines means an incoming enemy attack. 3 red lines means unblockable incoming enemy attack, etc.
When an enemy is aiming to shoot a projectile at you, above THEIR head will be a super thin green diamond. Just before the projectile hits you, you will get a green icon above your head that sort of looks like the top half of a set of eyelashes. When facing the enemy attempting to shoot you, hit
to shield bash the projectile back towards your enemy. This will damage the enemy a little and you must do this quite a few times to kill them this way.
You CAN hit them with your melee weapon a few times to knock their health down first. However, most enemies will switch to melee and come at you if you get to close. The rats that start coming around the games 3rd and 4th levels are good enemies for attempting this as they typically spawn in sets of four. 2 will rush you as melee and 2 usually hang back to shoot. If you dispatch the 2 melee rats and keep some distance you can just stand there knocking back arrows one after another until one of them eventually drop dead.