(Unique game aspects not part of story canon)
The U-9 is a creature created in the conceptual stages of Resident Evil 5. The U-9 itself bears a striking resemblance to the Uroboros creatures found in the final build of RE5, due to its composition of worm-like pustules, suggesting that U-9 itself was the basis for the Uroboros. Due to its towering size, it's most likely that the battle with U-9 would have involved mounted artillery, or even the Satellite Laser Device employed against Uroboros Aheri in the final release of RE5.
As seen in the image, U-9 is at least 80 feet tall, perhaps even taller than Uroboros Aheri of the final release; if the U-9 was in the final game, then it would be the tallest creature ever fought in the Resident Evil series.
Fighting this BOW would've likely included at least two stages: one where the monster is hunched over, and one where it stands upright. It was apparently going to be possible to blast the monster's head completely off; what this would've done is unknown.