The hammer robots:
Just roll behind them and hit the red button on their backs until they die.

The annhililator:
I actually don't know this guy's name but still, avoid the attacking robots and when the guy's bulldozer charges at you, dodge it and when it hits the wall, slash at the 3 red things on the back of the bulldozer until it blows up.

The portal guardian:
Dodge his punches and when he slams his fists on the ground dodge the falling staligtites and when he stops slash at his chest until he dies. When he dies, use the giant staligtite to get across to the other side of the lava lake. There will be a minimovie where you see the portal isn't there. Go through the passage to get to another minimovie where you see the portal being taken away by a train and you take the next train to Aku City. Go through the level until you get to 3 teenagers and then, you have access to the Aku City level.