To hunt in the spiritual dimension will require a method to enter the spiritual dimension and survive there. Both of these requirements must be met, or the spiritual dimension will remain off-limits like it has to many people.
Of course, there is also the requirement of having enough strength to actually kill the divine beast. This is a concern because it is expected that something that is further along the pathway than you will be stronger than you.
Legion is one of the few people that can kill people that are one level above them. So they are not worried about lacking the strength to kill a rank 8 blood beast while they are rank 7 killers themselves.
There is also the option of using their fishing rod to fish for blood beasts in the safety of the physical dimension. It will just be slow and uncertain compared to entering the spiritual dimension and having blood beasts appearing around them to hunt them and kill them.
