I think that it simply takes sheer deterination - I had to wait around the same amount just to find those annoying machea's in omega ruins. All I needed was 1 more to complete my entire collection - I ran to omega ruins an thought that they should logically turn up in at least half an hour - but nooooooooooooooo - 3 - 4 hours later I finally find two near the save point area. I haven't tried the reset trick althouogh I probably will in the fufture to test it's reliabilty. As for finding the black elementals - I found them to be one of the rarer fiends to appear along with halma's and varunas - just stick to it and think positively (you are getting a lot of gil)!