Translation feedback request


I'm redoing an older translation of mine and, within a part that I already
find very difficult (ie, I don't know if my sense of context is correct),
there's a line I can't get through.

え〜 めんどくさいな〜 どうせおれが一番強いにきまっているのに… まっ! いいかっ! バトルして正式に選んでもらえばいいもんなっ!

I get to "Eh〜 What a bother〜 And that even when it’s already been settled that
I’m the strongest… Wait! Get this!" and then I'm stumped. I see "do battle",
"select formally" "[make do favor]" "should" and an emotion indicator, but I
don't know how to put these together. For one because the subject/object isn't
clear to me, and for two because I think there's a mood swing (from bothered
to arrogantly joyous) involved and I don't get the reasoning for that.

The preceding context is that the speaker finds out he won't be a seed player
and has to participate in the preliminaries like everyone else. The one he's
speaking to is a friend. The friend responds with そうです! そのとおりですっ!

My guess is "I should do battle and get them to select me formally!", with もん
indicating his confidence he'll win. But I'd like feedback. Thanks in advance.




I feel your confusion started already from the "Get this!" part. This "まっ!
いいか!" is not "Listen" nor "Get this." It's something like "Oh well" or
"Whatever" in the sense of "On second thought, that (having to start from the
preliminaries) is not a big problem!"

Your understanding of the sentence you asked about is actually fine:

> バトルして正式に選んでもらえばいいもんなっ!
>
> (literally) If I battle and receive the favor of selecting me formally,
> (that's) okay!
>
> → All I need to do is battle and get them to select me formally (as a
> finalist)!

This もん is a little childish way of making an excuse or convincing someone (in
this case, convincing himself). It doesn't have to be translated, but it's
like "come on" or "you know". See: [〜ものだから 〜もの What are the
differences?](https://japanese.stackexchange.com/q/5648/5010)

