Once per turn, if this card battles an opponent's monster, during damage calculation (in either player's turn): You can send 1 Beast, Beast-Warrior, or Winged Beast-Type monster from your hand or Deck to the Graveyard, and if you do, this card gains ATK equal to the sent monster's Level x 100, until the end of the Battle Phase.
...You really run Big Koala just to Summon it with Birthright? (It's the only monster above 1800 ATK you could even be referring to.) Also, I assume "Blood Swamp" is referring to Sanguine Swamp, which destroys itself if you control any other S/Ts. Great combo there.
It is, if you don't plan to only use phoenix effect. I'm waiting it for my Sphynix deck. This monster effect is good enough imo. Importantly, it's damage-step/both player-turn/atk mod. If you really need to active phoenix effect, you can summon its from the grave and destroy it by effect on the field by the old set of card. This card just like free search card of soul without lp restriction.