Humanities for people with poor preparation?
Humanities prof in the US here. You are indeed correct that there aren't, generally speaking, research assistantships available at the undergraduate level in humanities. It's just not that easy even for a very capable, very motivated undergraduate to contribute much meaningful to the writing of a scholarly paper in most humanities fields. 

The best advice is to figure out exactly why you did poorly in the earlier course, correct the problem and then work very hard is a subsequent humanities course. You want to show that the one bad mark is an outlier. 

More generally, the best advice for success in humanities courses and applications to humanities graduate programs is to learn to write really well. If you can learn to write careful, focused, clearly argued essays that people actually enjoy reading, you'll have a definite leg up on the competition. Your bad marks might keep you from making the cut at some programs, but if you blow the writing sample away, you might become very competitive at programs that treat their admissions more holistically. (My sense, which is not very reliable, is that admissions decisions in the humanities tend to be a bit more holistic in general than in the sciences, since UG programs tend to vary much more in the humanities than in the sciences.)