Titan Dweevil is not that hard, just put all your non yellow pimins in the corner with the boss, and when he uses water pump, whistle.
Destroy the water pump first, then the shocker. Then it's easy.
I can't say any boss from the Banjo kazooie serie is hard, and I wouldn't say that boss from Mario galaxy is the hardest thing ever (even If I did die a bunch of times trying to get him without loosing a single life...)
There are a lot of RPG games bosses that are really hard.
Ruby weapon is really impossible, when you don't have knights of the round, I mean. If anyone has played "final fantasy tactics A2", there are some side-bosses that are impossible (5 units with level 99!).
I think older games have harder bosses, so we should seek in the SNES or NES games probably...
Ridley in super metroid maybe? Well, not THAT hard... But there are those 15% runs in metroid Zero and Fusion that makes some bosses nearly impossible. Oh yes, Fusion, the arachnid boss! That one was hard! Some bosses in Ocarina of time or A link to the past are quite hard too... Especially in the dark world.
Spekio level ** is hard too. If you ever manage to face him.
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