I don't believe any post-war UNSC frigate has taken on a heavyweight opponent alone. Visually, at least, we've only seen the Strident-class FF take on Covenant CRS-class light cruisers, smaller ships that are more like patrol corvettes in a human classification system. Engaging anything larger like RCS, CCS, or even CAS would have been in support of Infinity given her prominence in recent narratives. In any case it's a mistake to discount a frigate as a serious contender in fleet combat. There are a lot of metrics beyond specifications that make them often the perfect ship for many missions in the years following the war.