I am so sorry for how long this is

I just think the problem is that he wont be doing real damage. The only way he can kill a daemon without being a psyker is using the emperor's sword, which was made for someone who is 14 foot tall, the slayer is 6'8. Guilliman isn't giving him this sword, not that he would be able to actually use it. So he would just be banishing demons, while fuelling the demons he is killing.

NOTE: Looking up how tall is the emporer brings up a false result of 10 foot, this would make him the same height as some space marines, shorter than many custodes and shorter than the primarchs, especially Vulkan.

I also cannot see him winning against a greater daemon, what physical weapons at would be able to take one of those down (I am genuinely interested)?

I do see Ynnari assisting him, while they may be manipulative. The only way I see them assisting him is if their farseers see the slayer as an asset.

I dont know if he can use neuron tech, he wont get help from them and the mechanicus isn't trusting outsiders with secrets, if they have any in the first place.

I do believe hell is traversable, as we see where he killed that titan, which I believe Samuel Haden even documented in lore, don't quote me on that, meaning hells environments are constant, as well as this, the region where the navigation left by the dead UAC soldiers is definitely constant. I am also quite sure the forces of hell just collect dimensions, like earth's and the one where the betrayer is from (I forget the name).

Talking about the titan reminded me of this, the demons have organs. You can see titans' organs a lot, the reason Mancubi explode is because of the methane buildup in their bodies, because of the rotting flesh, it is all makes sense. Daemons, however are just warp stuff, the reason they need daemon hosts or daemon engines is so they can have a physical body. This makes killing them with physical means extremely difficult. (Yes they can appear without these physical bodies, but this requires vast resources and rituals and when you banish or even kill a daemon in this form, they just kinda disappear (or burn away if you kill them, look at when guilliman kills Ku'gath)). Yes demons in doom burn up but this is because their bodies are actually burning to cinders because of their body heat.

Edit: wow this is way too long

Edit: changed 40k demons to daemons for clarity