Midnight Fight Express

For this I was able to unlock it on level 10 "The S.S. Muggler," though I was also able to do this on level 6 "Abandoned Metro." I personally recommend going back to it after you've completed the game so you have all the combos and move sets. Additionally, keep in mind that if you die, your kills and kill variety for that level get reset back to whatever checkpoint you spawn at.

Below are the biggest tips I can give you:

1. Don't forget about environmental objects (throwables), which can be kicked or thrown with
2. Don't forget about your secondary gun by pressing , specifically the magnum and dart mine bullets as they are both lethal. The bullet type can be switched by pressing or
3. I recommend waiting until the boss fight to use your rope gun
   *** Spoiler - click to reveal *** as there will be two sailors (big guys that look like Popeye) that the rope gun works well against, along with the boss Arlena (lady double fisting electrified bats)
4. Push people up against walls and press so you crush enemies against the wall
5. Push enemies up against ledges to be able to press and throw them over the railing
6. Perform finishers by pressing the button when displayed over an enemy's head, usually when they are stunned on the ground or standing. I recommend trying to do one holding nothing (unarmed finisher), a melee weapon (weapon finisher), and a gun (gun finisher)
7. I personally didn't use any of the stun guns because they only stun an enemy, not kill them as far as I could tell
8. I don't know if this counts, but try to let the sailors take out an enemy to add to your tally