He doesn't have to be eaten whole at once, he can take a bite out of Doom and if he manages to defeat him, he can eat his entire body and stop him from returning
Pochita via his Devil Physiology also has Types 1, 3, 6 & 8 so that shouldn't be problem, including the fact that unlike other Devils Pochita keeps his memories upon death and can travel back to the battlefield within minutes
Doom Slayer would actually take this pretty easily though. His immortality negation would negate Pochita's Type 4 immortality, and none of Pochita's other forms of immortality would be able to bring him back. Types 1 and 3 obviously can be bypassed by the Doom Slayer since Type 1 doesn't actually do anything about unnatural causes of death (such as the Doom Slayer mutilating Pochita) and because type 3 can be bypassed by damaging Pochita so much that he can't regenerate, which is something that the Doom Slayer could easily do by utilizing his AP advantage and frying him with the BFG or something similar or cutting his heart up with any of his other weapons. Type 6 on the other hand can be bypassed by simply killing Pochita before he can possess another body since there's no indication that Pochita can possess another body while dead.
Type 8 immortality won't save Pochita either since reliant immortality, by nature, is simply a characteristic of another form of immortality. It's not a separate form of immortality. The page on immortality explains "The character gets benefited by 1 or more
other types of Immortality as long as a certain being, object, place, and/or concept or more may grant them those benefits." In this case, Pochita's type 8 immortality simply grants him type 4 immortality and nothing else as long as his concept continues to be feared by humans. It is nothing more than type 3 immortality with the condition of Pochita's concept continuing to instill fear in humans. This is rendered invalid however by the fact that the Doom Slayer's immortality negation negates this type 4 immortality in the first place. Since the type 4 immortality is the sole benefit of Pochita's type 8 immortality, his type 8 immortality is in effect neutralized.
Tl;dr Doom Slayer bypasses Types 1 and 3 by just damaging Pochita beyond his regenerative capabilities and bypasses type 4 and by extension type 8 (since his type 8 is nothing more than type 3 with a condition) with his immortality negation.
On top of that, Pochita doesn't even have any real wincons at all. Existence erasure won't work on him since the Doom Slayer's
acausality makes his existence a constant among all variables of all possible timelines and all possibilities. He MIGHT be able to die temporarily (that's debatable), but it is impossible for him to stop existing. Pochita would need causality manipulation or some other ability that lets him do the impossible in order to erase Doomguy, which Pochita does not have.
My vote is that the Doom Slayer takes this decisively.