This one fails to see a problem so long as each boss is unique, challenging, and fun.

There are only 6 Run 'n Gun levels and 3 Mausoleum levels, making 9 non-boss battle levels. The rest of the game is full of boss battles. 5 in Area 1, 5 in Area 2, 7 in Area 3, and 2 (11 if you want to count every individual boss during one of them) in Area 4.

Not quite all, but roughly two thirds of the game is boss battles, not that that's a bad thing.