If this card is Xyz Summoned with "Gagaga Magician" and "Gagaga Girl" as Xyz Materials, the effect of "Gagaga Girl" will reduce an opponent's monster's ATK to 0. This card's effect can the be used to inflict damage to your opponent equal to that monsters original ATK, and gain that same amount of ATK. If the monster had 2800 ATK or more, it can now attack for 8000 damage total.
An OTK can be done by using this card and "Mini-Guts". In order to perform this, a monster with at least 1900 ATK is needed in your opponent's field: 1900 (this card's effect) + 4300 (this card's ATK) + 1900 ("Guts") = 8100.
An OTK can also be done with this card with "Darkworld Shackles" and opponent monster with at least 3000 Original ATK: = 2900 damage (from effect) + 5200 damage (from this card attack) = 8100.
Keep in mind that this card doesn't only have to select cards that have lost ATK it can also pick cards that gained it. With this cards like "Copycat" or "Relinquished" can be used to drastically increase card's ATK power and still use this card's ability.
Given the above, this card is used best against a "Fortune Lady" Deck.
"Number C39: Utopia Ray" can be used to reduce your opponent's Life Points by 1000-3000, and use this card's effect to deal damage.
This card is a great counter against cards such as "Ancient Sacred Wyvern" and monsters equipped with "Megamorph", particularly the former, because it'll practically transfer the ATK boost of "Ancient Sacred Wyvern" to itself while damaging the opponent.
If your Life Points are lower, using your own "Megamorph" can do some serious damage when equipped to an opponent's high ATK monster. Plus, after the damage is inflicted, if your Life Points are now higher, the equipped monster's ATK is now halved, allowing yourself to do even more serious damage when attacking with this card.
Special Summon "Grinder Golem", "Lava Golem" or "Volcanic Queen" to your opponent's field to ensure that your opponent has a powerful monster to abuse with cards like "Darkworld Shackles".
This card can inflict major damage against a "Hieratic" Deck; when a "Hieratic" monster's effect goes off that Special Summons a Normal Dragon-Type, its ATK becomes 0.
A more useful way to use this card's effect would be to activate it (after it attacks directly) on an "Emissary of Darkness Token", effectively inflicting the same amount of damage twice, and doubling this card's ATK.