For any skillup (master or automaton) first find out your skill rank (A, B, C..) and your skill level. Sharpshot has an A- ranged skill and lets say your ranged skill is at 33, well under cap for a lv 40 automaton. Check this page http://wiki.ffxiclopedia.org/wiki/Skill_Caps and we can see that for an A- skill at 33 is equivalent to a level 10 puppet so you'll want to fight mobs level 10 and above.

If you have a particular mob in mind just look at its category page (http://wiki.ffxiclopedia.org/wiki/Worms) and pick something appropriate.

Recently its been changed that you can skill up on decent challenge mobs so you could fight a bit lower level mob but the skill ups will be slower.