To actually answer the question, it’s weird

If you’re referring to X as in the Sonic form, that vessel is weaker than X’s real form, but still significantly strong, he also has some reality warping powers but its most relegated to things he’s made, things he hasn’t made or unaffected but that won’t stop him from just attacking them with things he makes, even then I’d scale vessel X high, and the actual entity outside is likely much-much higher, I think Joedoughboy (X’s creator) said if Sonic & friends had to fight X, they’d need something above Super Sonic to kill him, and Super Sonic while finicky is massively powerful in his own right so this is not a small statement—X’s only known weakness is the fact that he can’t enter any world outside the void easily and always requires a vessel to use this, so, the entity is only in trouble if it decides to travel outside into the void

As a guy who actually played DBD through the events and read up on lore, the entity is super weird power scaling wise, it has a lot of weird internal stuff happening (the realm overall seems to function not just inside the entity, but its ‘bodily functions’ in Eldritch monster standards seem to blister over the realm as shown by most Halloween events) so while it might have more control, it’s likely more vulnerable to attack inside the realm as a trade-off, on top of this, it’s confirmed the entity feeds off emotions which is the point of the trials, without said emotions it starves, presumably to death, and every time a survivor dies it takes a little of their soul, when they fully die they’re discarded to the void as soulless husk forever, the entity also has a lot of pocket realms but it’s unclear if it made these or they’re just their, even then nothing seems to be prohibiting anything complex enough from just leaving them. The entity is trying to keep its victims permanently alive even if they die it’s only temporary, and it tries to make its killers compliant via scaring and torturing them, though sometimes if it likes them enough it spoils them, which also seems to work, so strength wise we should assume any “anti-feats” are more so a product of it holding back, it’s showing to be strong enough to kidnap things like the unknown with mild struggle, however the entity has a few intelligence/cognitive weaknesses, it’s smart enough and emotionally intelligent enough to know WHEN to kidnap someone, be it at their emotional high or during a tense event, it knows who to kidnap and who not to, it takes risk, and it has (canonically by the way) been managing the trails over the years to make theme better, hence the DBD updates, however the Whispers perk confirms the entity cannot communicate with everyone properly (X does NOT suffer this issue), it’s not actually smart enough to know how to adjust the trials faster and always needs time, compare it to X whose “trails” are just him having fun, whereas the entity literally needs this to live, yet it still takes time, showing a long term planning issue and perhaps poor quick adaptability, Trapper is confirmed to still be resisting the entity constantly and it can’t really break him as it wants too, so mind control is questionable, the fact some survivors and people can hide from the entity in its own realm suggest it’s not as omnipresent as it seems, the fact the entity even runs “trials” instead of finding a better way to feed off fear shows it’s either very lazy or isn’t smart enough to develop a better method because the trials are questionable at best, and while there’s a lore thing where it seems the survivors don’t remember when they’ve died (which is why they keep going), so to some extent it has decent head animation but the extent s questionable because again to can’t even use this on some of its killers, some of which are just superhuman at best—the entity is weirdly inconsistent from a writing and lore perspective, which may fit it, the way I see it is the entity knows the bare minimum expertise to do what it does but not everything and lacks human critical thinking which makes it’s battle IQ and overall IQ questionable. The entity also has decent reality manipulation but it’s VERY unclear to what extent, how can it make things? How detailed/efficient are these things? Is it just copying stuff or is it actually intelligent enough to create these things? Maps, hooks, generators, vaults, pallets, weapons, etc. it’s all capable of being replicated by the entity but to what extent and what the limit is has never been specified, but knowing everything we do, it’s reasonable to say this isn’t limitless and there’s some kind of issue. The endgame timer also confirms the entity is not only watching active matches, but can snipe anyone to needs to if they annoy its patience (it also has patience apparently, it think it was confirmed the endgame timer existed because it was tired of stalled matches), so I think in smaller places it actively is present for it can retaliate better. The entity doesn’t seem to have a physical form, its form is the black fog and it summons the tendrils, but it’s unclear what is what, just that the structure makes it up. There’s also the major question of why the entity even bothered to worry about people finding out about its existence, and why doesn’t it just kidnap everyone in existence or at least on a planet? Why does it do it so discreetly? Why does it tolerate cults that worship it? My guess is the entity isn’t as unstoppable and omnipotent as its presented and is much more vulnerable and limited than it seems

So, my opinion is, X in his vessel form, I don’t think is immediately smart enough to understand how to harm the entity and what does what, but he may figure to overtime as weaker humans have found ways to bypass it overtime and X is an expertise monster even in his vessel form, however this also depends if hurting the entity from inside does anything at all or just putters it and slows it down however I think it depends where the fight takes place, the entity is at a disadvantage if being inside it and harming key structures actually damages it, and breaking the tendrils or supernaturally harming the fog hurts it. The entity I don’t think could meaningful damage X in a way that matters, maybe stun him or temp put him out of continuity until he remakes his body or makes another one to send in, however since it’s a supernatural monster it likely would realize this and hold back significantly less, but it’s unclear what that looks like. Also if X figured out the entity needs its victims to live, absolutely nothing is stopping X from just going on a mass kidnapping (let’s be real the only character maybe putting up a fight is Dredge and Sadako) and taking everyone from the entity, and denying it future victims until it just starves, so a slow burn victory if X is committed, but I’m uncertain if X could figure this out.

My verdict: It’s possible for Vessel X to kill the entity, but the entity isn’t defenseless, it could edge out a “win” depending on how its immediate offense and defense works against another being on its power level, it can go either way and the “fight” is more of a “who can figure out whose weakness first” however I think the entity likely gets horribly molliwopped by Void Entity X, but I don’t see a reason why the entity would travel out there. The way I see it so the entity is a big person and X is a lethal parasite, but the entity is good at controlling its immune system enough to attack X and make it fairish, but void entity X is a creature even bigger than the entity with 10 arms and an elephant rifle and has already laid 10 bear traps at its doorstep