The first few levels seemed the only challenging ones, the hordes sent at you being a bit tricky to take down in time with a limited arsenal. After passing those, for normal difficulty , it was much easier, and on getting the flame thrower it became pretty trivial. The wall of constant rotating fire took out everything in my path very quickly, I only switched weapons and used the rocket launcher on the level it becomes available to you as it's pretty fun and useful there, but nothing else compared to the flame thrower until..... the flame thrower launcher for the last two levels!
My advice is to kill it with fire! Kill everything with fire!
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