Search by Subject Heading
In the sidebar, you'll find a list of subject headings that are especially relevant to epic fantasy. You are guaranteed to get results with these, because I personally vetted them.
GN Epic fantasy AND SU Mercenary troops AND ND Series
Now, some headings will get you too many results (SU magic), while others will retrieve too few (SU magic gems). For the former, I recommend combining appeals and subject headings. For the latter, well, you can always add OR SU magic rocks to boost your results.*