Well, when I played the game, I was also working on a FAQ at the same time (one of the first English-language FAQs available for the game, you can still find a copy at GameFAQs, lol, although it hasn't been updated since like 2000 and still has my college email), not to mention I played completely in Japanese, so that was probably the reason it took me so long. But yeah, if you didn't get Undine, Gnome, and Ifrit, do that before going to the Mines... there is an item that you get in I believe Ifrit's cave that will help you with the bullseye.
This game really has some of the best RPG dungeons this side of Lufia II though...