I was recently listening to the Hunter x Hunter anime, and during a conversation, a passing character that the protagonist meets states,
This struck me as a bit odd, as I expected to hear:
When I checked the unofficial captions I'm using, it reads, "Do your best to become a good Hunter."
I asked another Japanese learner his thoughts, and he theorized maybe it was a short form of "[stem] + に + [verb]". This would make sense given the caption being "to become" rather than "and you will become", but neither of us were really sure.
What is this usage of the verb stem, and when should/can I use it?