Stand in his front. When he raise his club run to his rear, turn back and jump. You'll land on top of his club. Turn back to face him immediately. When he withdraw his club you will being sent flying. Hit his head in the air.
Zoggot:
When you are surrounded by two tentacles, destroy one and turn back standing. This will block the attack of other tentacle. You should destroy all tentacles before you move forward.
Granny Summercamp:
Hit him when he try to crash you on his boom. It's not hard compared to other bosses.
Princess Vicious:
Walk to her melee range, and instead of attacking just jump. Behind her back, walk forward a bit then turn around to hit her back.
Phantom Clone:
Use heavy attack at every chances. Easier then you may thought.
Hedgehog from Hell:
Jump and dodge his rolling attack, land near its tail to hit it. If it spilled some spines look at the ground, avoid the spines and jump to dodge rolling attack simutanously.
Duke of Hell:
Kill his minions to get max rage. Then look at the ground to dodge his star (the boss is clever he know the meaning of locus, you have to dodge that carefully). Stay under the wall until he stop firebreathing. Jump in front of him and use special (max rage needed). You will clash sword with the Duke. Win the clash, then jump up to hit his head when the Duke is stunned.
After you beat the Duke you can go back to Barbarian Stronghold to battle in survival mode. If you killed enough enemies you will unlock the secret cheat level.
I dont know if the game ends here, but I have yet to find out The Secret of Steel...