Chain this card to any attack negation for free Life Points. In particular, "Draining Shield" before this card gives you equal the ATK of the attacking Monster in Life Points. Note that if Enchanted Javelin is used on it's own, it will only work as a negation to a monster's attack because it is not stopping a monster's attack. It only increases your Life Points by the same amount as the ATK of the monster. At the same time, the monster is still attacking your Life Points. Therefore, no damage is done to you, but there is no increase in your LP, unless you have a monster in defense position.
When deciding whether to use this card or "Draining Shield", consider whether or not you'll want an attack to connect most of the time. While "Draining Shield" tends to be a more solid choice in conventional decks, Enchanted Javelin wins out when you consistently need to take damage or have a monster killed in battle, because it gives you valuable Life Points in the midst of suicide tactics. Also, if "Numinous Healer" or "Nutrient Z" are employed alongside Enchanted Javelin, you have a shot at getting more Life Points than you would have with "Draining Shield". Using this card over "Draining Shield" can also allow you to summon "Gorz the Emissary of Darkness" or "Tragoedia" while effectively taking no damage.