The hardest boss in FF IV is no-question Brachioraidos. You can beat him as many times as you want, (if you can at all) and get a hero's shield for each of your party members. F.Y.I. Best shield in the game.
Brachioradius was pretty easy...Zeromus EG was hard on the 2nd time killing cause it got glitched....his attack normally 1000-2000 but no...his final attack (tidal wave tornado) did 9999 dmg..and when he healed himself he hurt himself.
I thought Brachioraidos was harder than Zeromus EG actually, I told myself after beating the first one that is wasn't worth the time for more hero shields.