Any help won't really do much (the enemies are in another world just visiting briefly at times), and be more liable to just have the helpers die if they do stumble onto the enemies. But if he truly wanted their help he could still ask for it without being made a hunter.Genuine question, considering that in a previous reply, you implied that this would be faster: Do you think that disregarding the help he's getting from the people with the most know how about the current state of the world behind the scenes (and probably even more information that would be useful to him and his goals), antagonizing the dude who just proved that he's willing to violently disrupt his life if he's not going through the proper procedures, then lawyering up and taking said dude to court, would be the quickest way for him to achieve what he wants?
If so, how?
The antagonistic dude would be very unlikely to actually go too far, but even if he does, mc could easily clean up the hunters guild in an evening considering their locations are public. All without leaving any traces for evidence against his owner's body.
Lawyering up is not exactly needed, as if you want to give the guild guy legal issues you simply need to release some evidence. Because murder, assault, or kidnapping (which is what the guy is threatening to do, either to mc, or maybe those around him [X to doubt]) is not something you take to court yourself. It is just that the quickest and easiest hands-off approach to getting the actual processes started is to have the lawyer do them as your proxy, so you don't need to waste your time on it yourself (beyond maybe testifying as a witness on some recording, though even that wouldn't be needed considering mc's capabilities to easily get hard evidence).
But no, to answer your question, I don't think that would be the quickest way, just far faster than becoming a tool/slave having to obey a guild, whose main goal would be to keep him collared and busy with their tasks and not able to run free (which he needs to chase the demons). Since that alternative has a speed of 0.
I think the easiest and fastest way for mc would be to go murderhobo (though given the mc/genre we are dealing with, murder won't happen. At most some shounen beatings) on anyone going beyond 'merely' threatening him. But that is all besides the point, as (aside from the sheer fact that this mc won't do that) if their issue is that he is a dangerous threat, then they wouldn't antagonize him into actually being a dangerous threat. Particularly not one with no recourse but systematic violence (like if a hostage is taken). So they won't touch him either way.