This game is amazing, i finished both games, TFE and TSE on mental, this one in my opinion is more challenging because of the increased enemy roster and the verticality of the levels and the enemy placements.
The Grand Cathedral is a masterpiece and theme is amazing, no wonder it got a remake in SS4, and the level doesn't disappoint in terms of difficulty, especially on Mental and Serious difficulty but it's a great level, and a perfect conclusion to the game, it leaves a lot of possibility for the next game, where will Sam be going to in his quest to kick mental's ass, and hopefully this time as Sam knows how to pilot a spaceship, it doesn't crash into a crate-bus.
All in all this is a great game, 10/10