Hunt Prey takes a single action, as can be seen from the symbol next to the name. The rules text does not specify any limitations on how often the ability can be used, so you can designate targets as often as you like, provided you're willing to spend a single action each time.
Your best bet is probably a Monster Slayer using a Hand Crossbow, with the Crossbow Expert and Sharpshooter feats
I'll start by pointing out that I don't think that "assuming attacks hit" is an especially effective way to gauge character damage output. Many methods of producing damage, like the Sharpshooter and Great Weapon Master feats, specifically ...
301.5 damage total, 55.5 damage on most rounds
Given that we assume all attacks hit, any serious attempt must include either Great Weapon Master + Polearm Master or Sharpshooter + Crossbow Expert so at to maximize the number of attacks and the damage on a hit.
None of the many reasons we might favor the SS+CE over GWM+PM (Archery Fighting ...
The UA Ranger's wolf companion
To answer your questions:
"The average of 2d8 should be 10. 10+2 is 12." This explanation by kriivi via a comment is better than my original attempt, so I'm going to quote it here:
Expected value is linear, which means that the sum of individual die values will do. In the case of d8, the expected value (or more intuitively,...
The average (expected value) of 2d8 is 9, not 10.
The rules for monsters and PC-s are separate. There is no such thing as a level 1 wolf (unless you awaken it to sentience and it gains a PC level). You are correct that its hit die is a d8.
To me it seems that either they miscalculated or are using different rules than you are (...