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If you ask me "Fight with me once more" and "Fight me once more" have excactly the same meaning, but a different grammatical contruction. As far as I know neither of them is wrong. If you really want to know why the translator decided to translate a certains sentence in a certain way, you'll have that person him/herself.
I'm not sure if there is anything to do with it, but fight with me takes up more space than fight me. The best way to find out is to contact molo. (He is such an excellent translator, by the way.) You could PM him or make a post in his thread: here is the Link
thank you guys for help.And molo's replied me.So nice
僕と戦いでください translates literally into 'fight with me'. と in contexts like this one usually means 'with'. A translator who is not skilled in English, or who translates very literally will ignore the opportunity to simplify the sentence by removing the 'with'. Because 'to fight (someone)' already implies participation, 'with' is redundant. Two main possibilities arise; shortening the translation did not occur to the translator, or, as mikkih suggests, there was a good reason for the translation. 'with' also tends to imply co-operation (fight beside me once more), so if the character is talking to a friend, this is the reason.
All considered, the translator might not have thought of removing 'with', or if they did, they had reason to preserve the 'with'. Either way, the question ultimately falls upon the original translator's thinking process.
Hopefully molo helped you out