When should I use a verb stem by itself?

Question

I was recently listening to the Hunter x Hunter anime, and during a conversation, a passing character that the protagonist meets states,

頑張って、いいハンターになりな

This struck me as a bit odd, as I expected to hear:

頑張って、いいハンターになるな

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?

Answer 1

なりな is an imperative form, with 〜な derived from 〜なさい.

From 大辞泉:

２ 《補助動詞「なさる」の命令形「なさい」の省略形》動詞・動詞型助動詞の連用形に付く。命令の意を表す。「早く行き―」「好きなようにやり―」

Be careful not to mix this up with なるな, which can itself have multiple meanings.