Preferring to fight the player in this diabolical contraption, the Dark Machinist Child's is housed in the white cockpit, at the back of the room, armed...literally...with buzz-saws, claws and screw drivers, the machinist will try and lunge the player with each of the four arms and it will hurt if you're not careful.
Easy/Normal: The most straightforward means is to use your Electrotool at the Machnist while dodging the arms. After a good lengthy hurt, he'll go down.
Hard: The spawning chassisbots would be a pain to even bother using the electrotool safely without being attacked. Take down the spawning bots before focusing on the machnist with your Plasma Autorifle's grenades. It doesn't react well to a well-placed grenade.
Tip: You may also could try hiding in the alcoves to avoid being struck by the arms and shoot from there but be aware of the spawning Chassisbots while doing so...besides you can get a steady supply of plasma grenades.