In order to use this monster's ATK-reduction effect, it is necessary to take effect damage on the same turn. This can be easily achieved through the effects of "Dark Contract" cards, or effects like "Ring of Destruction", "Destruction Ring", or "Tremendous Fire" (the former two, as Trap cards, will also let you use the effect during your opponent's turn).
You can easily Fusion Summon "D/D/D Dragonbane King Beowulf" by Tributing this card or using it as a Fusion Material to send it to the Graveyard. Then, use this card's effect to send "D/D Necro Slime" from your Deck to your Graveyard. Banish both this card and "Necro Slime" with the effect of "Necro Slime" to Fusion Summon "Beowulf".