Those bosses can be a pain in the wassname, can't they? There're a few tricks to get rid of them, though.
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Shriney Guard (Monkey Shrines):
This one is fairly easy for his size. Jump on the highest block (in the middle of the screen), and from there butt-bounce the head of the Guard while he's standing on the left or right of the screen, then jump back on the block to not be over-rolled by the Monkey. Do that three times and he gives up.
Joe-Head-Joe (Skullmonkey Brand HotDogs):
Easiest thing is to use five Klaybullets on him - but you wouldn't want that, right..?
The more complicated way would be: Ducking from the fire he spits and jumping over his Eyeballs (Fire and Eyes, hehheh. Gettit? Uhhh..) until he burbs out a little blue bubble - you can jump on it and from there on Joe's head. You'll have to do that five times, only once with each blue bubble. To make it a bit easier there's a pattern behind it:
1) Fire, Fire, Bubble!
2) Fire, Eyes, Fire, Fire, Bubble!
3) Eyes, Fire, Fire, Eyes, Eyes, Fire, Eyes, Bubble!
4) Fire, Eyes, Fire, Eyes, Eyes, Eyes, Fire, Eyes, Eyes, Fire, Eyes, Fire, Eyes, Bubble!
5) Fire, Fire, Fire, Eyes, Fire, Eyes, Fire, Fire, Fire, Eyes, Fire, Fire, Fire, Fire, Fire, Eyes, Fire, Bubble!
Glenn Yntis (Ynt Eggs):
He isn't half as hard as it seems at first. You have to shoot his feet to make him raving mad and drop his guard, then shoot his chest. And that five times. But he sure uses his claws to try to snap Klaymen back! To avoid them look where his smaller claw points - that's where he'll strike with the big claw.
Monkey Mage (Castle de los Muertos):
Either you use a Phoenix Hand as soon as the Mages' shield turns off, or you'll jump on his head (I really prefer that). For both the timing has to be quite good. Stand still on your platform until you see the magical shield of the Monkey flicker in a brighter orange. Then it's time to high-jump (or shoot) on the head of the guy (you need to get him right inbetween his shield vanishing and him releasing a spell to smoke one of your platforms), and then jump right back to a platform to wait for the next attack. You've gotta do that five times.
Klogg (Worm Graveyard):
Klogg is actually, uh, one of the easiest bosses, if you ask me. Dodge all the spiky balls Klogg throws at you, but sometimes his machine produces a smooth, spikeless ball - catch those and shoot them right back at Klaymen's evil bro! Where to wait for the smooth balls is on you.. you can either wait near the middle of the screen, or you can do it like me and always be right in the middle under Klogg's machine, where the spike-balls will miss you and you can easily catch the smoothies. Five times.. five times and Klogg is defeated, yet again.