Soldner-X: Himmelssturmer

You’ll need to have the Hidden Passage trophy to unlock the fifth world. Once you get there, this is easily the hardest of the five levels. Towards the end of the level, killing certain enemies will release anti-powerups that slow you down or instantly kill you. Once you get to the final boss, amazingly enough, you’ll meet your old friend from Portal (if you’ve played through The Orange Box, that is). You can only damage the cube after it flashes hearts on the side of it.