One problem I noticed is that if the protagonist is promoted too quickly, the boss is too easy to die, and may not be able to see the new vore animation or additional story, and then there will be a feeling that the boss is too weak to stir up the desire to win, so I recommend adding a boss below a certain amount of health will cause a short period of invincibility (within a battle).
An interesting point to add is that every time my health was very low, I relied on the slow-moving Slime and fought against the boss in the dying.