I played it first time without guides (cause I couldn't afford all those yet) the hardest boss was the The Cosmo Canyon Boss, Materia Guardian, Demon Gate, the two headed dragon on the northern cave (the one that uses ice on one head and fire on the other and uses quake once in a while) and of course Ruby Weapon (Emerald was kinda easy cause I used the W-Item thing (nobody told me about that, I just found it out myself). Now I use the guide to keep track of my materias, items accesories whatsoever. I don't need it for bossess anymore.