Typically when using a melee attack you overheat the ARS suit and have to wait for it to cool down before you can attack again. However this cooldown can be bypassed by attacking whilst wielding the Disk launcher which has a unique melee attack.
Load up Act 2:1 - River and move around the corner and open the weapon box. If the box doesn't contain a Disk Launcher then restart the checkpoint and repeat until you have obtained one. Once you have a Disk launcher move forward and allow the enemy to kill you, this will save your loadout and allow you to restart the checkpoint with the Disk Launcher.
Once you've acquired the Disk Launcher and restarted the checkpoint boost under the truck on the left and proceed to melee all of the robots. Then reload the checkpoint and repeat as necessary to reach the required 100 kills.
For reference, after my initial playthrough on Casual I only needed one run here to reach 100 kills.