Well if you are like me who plays solo becasue you also have no one else to play with I'll share my method of getting the acheivement while going solo.
First you need to unlock NegaScott by clearing the main story mode with all starter characters, this does not include Knives.
Then you have to get NegaScott's lvl to 16 to learn his first tech attack and max out his Strength. Now set the game on easy and head to the 1st stage. Once there just block until you get a clear shot with his tech attack which should instanly kill the enemies if you maxed out his strength, and be careful using it near any object that can bounce back and hit you.
*the purple hair'd dudes might be tricky to hit since they love to dodge
*try to wait until the dogs are on one side of you before going for the kill
*be very careful when you reach the underpass so you don't get hit by cars
*and very cautious of the guys who love to throw stuff at you