I'm not sure on the specific terminology commonly used in the game, but I'd say something along the lines of
すみません。name of boss戦の手助けをしてくれる方いませんか？よろしくお願いします。
It's been a while since I've played an online game in Japanese so the politeness level may be higher than necessary, but that shouldn't really hurt.
A couple things are off here. First, "[enemy]と戦う" is grammatically correct, but it sounds slightly awkward here because you use it right after "一緒に" which also often uses と to indicate who the "together" is with. While this isn't wrong, I'd recommend reorganizing the sentence a bit to avoid such ambiguities (though in this case I think the reader would know what you meant).
Secondly (and more importantly) "戦うたすけて" is grammatically incorrect because you're trying to verb a verb and the result is akin to saying "can someone fighting-help me with this boss?" What you need to do in this case is nominalize the first verb so that you can act on it with the second (in English you could say "can someone help me [to] fight this boss?").
Japanese has two primary grammatical structures used to nominalize verbs: の and こと. There's a lot of nuance that goes into deciding which to use, but generally speaking we can use の to nominalize verbs that aren't being used to describe specific occurrences. We do this by adding の to the end of the infinitive form of the verb and then supplementing it with the particle appropriate for the verb that follows it. In this case, that would look like
I used 手伝う here because 助ける sounds like you're asking someone to save you from having to fight the boss, but grammatically speaking they're interchangeable. I'd recommend looking up how to use の and こと because they're very useful and they're common enough that most first/second year textbooks should include lessons on them (I know Genki does, for example). Prolly other stuff online, too.