I actually completed all the levels - reading my comment again, it sounded like I didn't finish. That said, certain levels felt awkward to complete; like I had to hack my way through them rather than use the mechanics as they were probably intended.
For example, to beat the level below I pushed the top barrel onto the first enemy so he couldn't see me. This also prevented the second enemy from seeing me when I jumped to shoot the barrel through the slit. In the end, I never figured out what the purpose of the first flaming barrel was...