The Surge

This achievement requires you to kill all 5 Smelter Bots around the game.

The first is located at the start of the game by the med-bay, instead of going right around the outside of the building to get to the P.A.X fight, keep going towards the right, behind some pipes will be the first bot, beware as these guys can one shot you early on so it may be worth to come back later.

The second and third bot spawn in the middle of the recycling floor, they will be on their own platform separated by a small gap. You can either jump the gap or go upstairs and activate the bridges. There does seem like a bug with these as on my second and third playthrough of NG+ one of them didn't appear but the coin was there.

The fourth bot in order will be in the old biolabs area. When you finally get outside to the right of the Large Greenhouse facility will be a small gap leading into the toxic waste. The smelter bot is sitting here, beware as the gas does increasingly more damage the longer you stay in as well as some drones will shoot you. The good thing is there is a first aid machine at the top of the stairs where you come in from so you can plan ahead to heal here and then return. Recommended to come back later when you have good weapons and a lot of healing.

The last smelter bot is hard to miss as when you come back to the main facility from the biolabs you will see a large bridge where the enemies are attacking each other. Using your new drone powers you can open the gate behind these enemies and reveal another guard and smelter bot in the corner.

Final bit: these don't have to be on the same playthrough, if you kill 4 on one playthrough, as long as on NG+ you kill the one you are missing the achievement should pop.

Footpickle's guide is really good, definitely go check out his solution and give him a thumb up.

Only thing I feel he forgot to mention was a strategy. I'll also include a video showing you the location of each bot as I feel that's easier to follow.

You want to grab Smelter bot's attention and make him do his spin attack, where he spins his arm for about 3 seconds. After this, just quickly jump as close to his body as possible. Try to avoid his arm while doing so because he has this "poke" attack. Once you're close it's all about circling left or right and hitting his sides. He also has these 2 attacks when you're up close. Slams his body down, deals a massive amount of damage if you're really close, easy to predict, arm stops following you and he lifts himself up a bit. Once you see this, jump back and once he's down you can continue. The second attack is a gas, not to worry it only slows you down a bit and prevents stamina regeneration.

For Smelter bot 4 you want to wait after you rescue Dr. Chavez. "Dog" robots will be replaced by flamethrower guys. Also try to have full liquidators set, as that will prevent gas damage. There are 2 flying bots on your left when you enter, make sure to kill them by luring them out before trying to kill the Smelter bot. Just as Footpickle said if you're low on health run back up the stairs and replenish your health.

Here is the video guide, all credit goes to NonstopTrophies. If you used this video please give him a thumb up.

You may want to kill the 2nd and 3rd Smelter before completing the quest where you have to give the woman a staff weapon. It could possibly void the achievement for you as it did for me, as she kills one of them herself after you give her a staff. A coin will still appear where the smelter would be. Goodluck :)