syntax - Sentence structure and meaning

here is an extract from a manga that gives me troubles : (|| = column breaks in the manga)

ただし||お前達の相手は||私一人でいい||一対一で闘い||私が勝ったら||次の者が||また　私と闘うのだ...

First : can you confirm that there are 2 different sentences :

ただしお前達の相手は私一人でいい。

However, I will be your only opponent. (???)

And

一対一で闘い私が勝ったら次の者がまた私と闘うのだ...

We wll fight one vs one and, if I win, the next person will fight with me again....

And that 闘い is equivalent to 闘って there? (Which sounds weird to me because I learned that it had a litterary use only and the character is speaking informally the rest of the time, using だ instead of です, 楽しませてくれ, instead ot 楽しませてください and so on...)

Assuming I'm right until there, I don't get what the first sentence means (especially why いい is used instead of a simple 私一人だ, but I found a lot of 私一人でいい occurences on google so...)。

Or I'm wrong and で is the てform of だ and いい modifies 一対一で, and I don't understand it either : Fight a good one vs one?)

Or, (thinking while writing), I'm wrong again and 闘い is just a noun and not the equivalent of 闘って and there is an を particle ommited after 闘い and いい modifies 一対一で闘い and then :

ただしお前達の相手は私一人で

However, I will be your only opponent, and

いい一対一で闘いを私が勝ったら

If I win a good one on one fight,

次の者また私と闘うのだ...

The next person will fight with me again...

Answer

That is clearly two sentences and you divided it correctly at the end of 「[私一人]{わたしひとり}でいい」.

「[闘]{たたか}い」 is the [連用形]{れんようけい} of the verb 「闘う」 and it has the same meaning as 「闘って」, the inexplicably popular form among Japanese-learners. 「闘い」 is surely more formal than 「闘って」 but it is NOT for literary use only as you seem to have learned incorrectly somewhere. 連用形 is used in fairly informal speech as well.

「ただしお[前達]{まえたち}の[相手]{あいて}は私一人でいい。」 is 100% natural. You could replace the 「でいい」 by a 「だ」 if it were only for grammatical correctness. Meaning-wise, however, 「でいい」 adds much more. It expresses the speaker's agreement to the fighting rules that could possibly be disadvantageous to him. In other words, the speaker is already very confident of his victory as he speaks.

「いい」 does not modify 「一対一で」 as the first sentence ends with the 「いい」. And no, 「闘い」 is not a noun here; It is a verb.

"However, I do not mind being you guys' only opponent."

"We will fight one on one (at a time) and if i win, the next guy will fight with me again."