Featherwick, for me Fusion Dragon was always pretty simplistic - Protect/Heal/Djinn, rinse and repeat - and I was always level 32 when I got to it. Deadbeard, however, definitely is harder than Fusion Dragon and can be harder than Dullahan in his own right - Deadbeard is designed to kill you off ASAP, while Dullahan is designed for a prolonged affair.
There was that one time I fought Deadbeard at level 23 and later Fusion Dragon at level 28. Oh man, that was a hard but interesting playthrough. (I actually always am level 23 by the time I get to Deadbeard, but I frequently level-grind just to get Break before I fight him. Those buffs are evil.)