All the Greater Fiends were. 
Maybe they're awesome bosses on Mentor or Master Ninja though.
The second Zedonius fight in the middle of the lava was one of the worst boss fights ever. I had to get lucky and manage to keep him on the ground long enough to kill him. The first few times, he just took off into the air, and then I wasn't fighting him; I was fighting the lava, the camera and his stupid minions.
That fight was cake. In fact, I haven't had a problem with any of the bosses since like Chapter 4 or 5. Maybe it's because I've simply gotten much better at the game, I dunno, but now I never die more than once or twice at any of the bosses and am always loaded with healing items. The scythe is simply a monster of a weap. Just keep pressing triangle and you'll kill anything. Jumping + triangle where he does that double attack in the air works great as well. I'm finding the whining about the bosses in this game, both in here and especially in that GAF thread, unwarranted. I'm not even good at videogames and these bosses are going down easy.
Two armadillos boss fight? Got em on my second try. All I did was concentrate on one and then taking out the other was really easy. Methods used were reverse winding when they rolled (you can completely dodge their rolls doing this) and then combo'ing them with the scythe. A single strike to their heads does huge damage, but hitting those adorable little legs does the trick as well. As for them exploding afterwards? The first one I had a Talisman of Rebirth on and when the second one was about to die I was like how am I gonna survive this atom bomb that's about to be dropped? Then it came to me, just use Inferno ninpo when he's about to explode and you're invincible through the blast.
Worm boss early in the game? Just shoot him in the head with charged up arrows and either Inferno ninpo to block the flying fish or Lunar Staff strong attack. All the bitching I heard in the GAF thread about this boss was ridiculous. What a bunch of whiny, pale, pasty, skinny little ninja dogs those guys are.
Genshin? The first Genshin was harder than the others, but not that difficult. The second and third Genshin fights are very easy thanks to the range and power of the scythe.
Volf? Not even worth mentioning in terms of difficulty, both fights.
The underground rolly polly? All I did was use the Phoenix ninpo to stun him and then attacked with Falcon's Claws. He died after about four passes.
The two flying, golden dragons? Like Volf, not worth mentioning. Charged arrows took them both out in a few minutes.
Elizibet and whatever other bosses I'm forgetting were all easy as well.
Btw, I'm playing on Warrior diff not Acolyte.