Speed Limit (UP)

Turns out the conga line guys are the group of enemies following you on Stage 1, so you need to kill 100 of them without getting killed yourself. You CAN get this on Easy Mode and I highly suggest that. My method is pretty slow but works nicely since Stage 1 isn't very long so you need to get as many kills as possible whenever you have the opportunity, I simply walked forward until the line of enemies disappeared from my sight, then stopped and let them see me, the first guy with the shield will crouch down and you can get 2 kills every time you do this, of course watch out for the normal enemies in the level, if you get killed just restart from the main menu.

Video of me getting the achievement:

Easier way to get this achievement (thanks to a comment I saw on a YT video).

Select easy, make your way to the end of the stage (the last carriage, but do not let the rocket guy spawn). The conga line guys will stop just away from the door, kill 2, run back into last carriage (again don't let rocket guy spawn), run back in, then out (twice, to avoid the grenades), then kill another 2 guys - repeat.

It may take a lot longer, but overall is a safer way to do it!