In my opinion, whether an animal's soloable or not should really come down to whether it's possible to fell it if you A: know its anatomy and are skilled enough to land a critical hit (in the literal, not popular, definition of the term), or B: prepare well enough in advance of the hunt, and are lucky enough, that you can manipulate the prey into a situation in which it's impossible for it to survive.
Bear in mind that hunting's not just going out into the bush with a bow/crossbow, sighting on an animal, and shooting it. There's other ways to go about bringing an animal down. The use of traps, mild poisons, geography, unique equipment and the like will all alter the strategy a hunter approaches the hunt with. Could a single hunter solo an ursaphant with nothing but a bow, some arrows, and high agility? Probably not. Could he poison it with an arrowhead coated in tranquiliser from high ground, track the creature's blood trail to its eventual point of collapse, and slit its throat? Significantly more likely, if he knows the dosage needed, the lay of the land, the anatomy of his target, and how to use his weapon properly. Whether a creature is soloable or not should, in a lot of cases, really come down to preparation.
Obviously this is a stock example, ursaphant hide might be tough enough to resist arrows. Or there might be some other complicating factor that would make this less viable. But if we're not being creative, a lot more in the game is going to seem impossible than what actually is.