First off, the Saturn Sigil is for Kimahri, which is obtained by catching butterflies in Macalania.

If you are having trouble catching Fiends for Auron's Sigil (Mars), simply catch more. It isn't just about cataching the sufficient amount of monsters, it is about catching them ALL. Pokemon!

Also, you must catch ten fiends (amount of a fiend) of 10 different fiends (such as Bombs and Dingos) from 10 different areas (locations).