Kid Chaos? Very manageable in the first half of the game (repeated goes and you should get through that level 2-2 without breaking into a sweat), then it gets harder. and harder.
For a laugh, type in the cheat 'hardasnails' and select the last level, 5-3. It is utterly impossible, unless you know the game inside out.
Level 4-3 (the Techno Fortress) is also insanely difficult.