The effect targets 1 attacking monster. Also, it does not target the monster whose ATK is increased.
You can even activate this card when a monster with 0 ATK attacks.
Negating the attack and increasing ATK are treated as being done at the same time.
In "1 face-up LIGHT monster you control gains ATK equal to that target's ATK, until your next End Phase", the text "until the End Phase" should read "until the end of the turn". (This is effective from July 10, 2014.)
You cannot activate "Lumenize" if there is not a face-up LIGHT monster on your field.