Brazekool
What lets me down is Boss' that have to legitimize their place by either having uber-HP levels, uber-defence stats; uber-healing abilities; or the classic, fight me for a little while, then fight my minions while I restore full, then fight me again and repeat, style of battle. It'd be nice if the boss was of similar stats to the player character and the reason it wins is though legitimately superior strategy.
This is why I enjoyed fighting Caius (in human form, since his Bahamut form is more like what you described). He has a regen ability that I don't think comes off, but that's about it. He will buff, but you can dispel it. He hits hard, but there are ways to cut down the damage. He doesn't have incredible amounts of HP. The fight keeps you on your toes, because if you stop to heal too long, you'll find his regen ability has fully restored his HP.
Of course it's really easy to over power him just by leveling up. But that really applies to any boss, doesn't it?