As someone in a math-heavy field, the advice one is generally given if one's preparation for potentially going into graduate school is poor boils down to the following:
Do better in a class that succeeds the class you did badly in. The example, I think, was something like "if you did badly in algebra, take algebraic topology." But in undergraduate humanities programmes, there isn't necessarily as obvious a succession unless one looks at course numbering and intensity (e.g. presumbly a small seminar in the high 200's will be more intensive than a large survey introductory lecture class).
Do RA work for a year or two after one's undergraduate years (which might allow one to take discounted classes). But I am unsure if RA work is as plentiful among, say, scholars of Marx or medieval theology as it might be to biochemists or economists. Naturally, it would seem that there's probably some demand for translators or archivists, and I know people who have interned at museums as researchers, so maybe I'm not aware of something.
So I feel like it's hard to map advice in more overly structured programmes that may also emphasize things like hard data more to the humanities.
Is this true, and if so, what would be good advice for such situations?