The correct way to parse "就打赢了" in 是个很普通就打赢了的对手哦?
&quot;是个很普通就打赢了的&quot; is an adjectival phrase for &quot;对手&quot;

是 - is (verb)
个 - a (classifier for the noun 对手)
很普通就 (just easily) 打赢了(defeated)
的 (adjectival phrase marker)
对手- opponent [noun]

就 here means &quot;just&quot;
&quot;打赢了的对手&quot; = &quot;defeated opponent&quot;
&quot;是个很普通(就)打赢了的对手&quot; = &quot;is (just) an easily defeated opponent&quot;

'普通就' is short for '普通地打就' (just fight normally)
Other example of '就' functions as 'just':
一美元(就)卖了 = sold (just) for one dollar