Maybe hold down the jump button for max jump height? I'm tired of jumping at max height straight into a hole but you know what they say "a few tricky jumps make the game more challenging" It's not challenging just more or less poor game design. Now if you'll excuse me I have to get prepared for the negative votes that are incoming.
Ok so let me get this straight. There was a party that a witch didn't get invited to so she turns all the party guests into giant rat monsters then I go on a quest to murder her?
Walkthrough of a sort: For the first two missons use your assault rifle as much as possible collect all the cash you can. If you start to run low on ammo it's ok to buy some. As soon as you can junk the assault rifle and buy the smg and make sure you have extended mags and fmj on it. Use that for the remainder of the game. In the next to levels after that try to get to level 20. Buy the rocket launcher as soon as possible for the bosses. The extended mags for it aren't needed buy come in handy sometimes. Tips: keep your distance from monsters, make sure you kill all monsters before engaging the bosses. Not sure if this will help but this is what worked best for me.
 "The End"){kind=icon}
 "The Last Door - Chapter 1: The Letter"){kind=icon}
 "Ghostscape"){kind=icon}
 "Morningstar"){kind=icon}
 "Nightmare Runner"){kind=icon}
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 "Space is Key 2"){kind=icon}
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 "REW"){kind=icon}
 "Chuck the Sheep"){kind=icon}
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 "Scriball"){kind=icon}
 "My \"Dear\" Boss"){kind=icon}
 "Blockage"){kind=icon}
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 "Musical Brick Block"){kind=icon}