Syndicate

In chapter 20 you will fight the final boss and his two agents.

Run to the right and take cover behind the pillars.

Disable one of the agents armour by holding LB on them and shoot their health down, do the same with the other agent.

Then shoot the boss and he falls down with the elevator.

Drop one of the agents to the floor and the other will run up to her trying to revive her, then drop the second agent, you can now click in the RS to kill them both.

even though the video by iNTiMiDaTe xX does a nice job in showing you what needs to be done, it doesn't offer much strategy (which lead to me dying a lot). if you don't have good weapons you have assault rifles to your left and behind you in the far right. there are also two shotgun racks behind the raisable pillars. i suggest staying behind these pillars and alternating between the two in the far rear of the room. the twins will constantly try to flank you, while merit from the top of the elevator will shot his minigun and lob grenades. since both twins have shields you want to want to patiently wait behind the pillars until one tries to sneak up on you. breach her shield and pump them full of lead while on Dart mode until the shield is gone. repeat the process for the other one. in case the second twin is two scared to come after you and the first one comes back, breach them again until they are really low on health, but DON"T kill them. in between all of this shoot Merit a few times, nothing fancy, just so he isn't attacking you or providing a shock shield on the downed twin. when you have one of the twins done to really low health. proceed to get the other twin to really low health too. now, these twins only have to be in proximity of each other to revive one another, so keep that in mind as well as keeping Merit lowered. ok, so by this point both twins are at really low health, and running around trying to find you. you're still behind the rear pillars, playing it safe. when one twin seeks you out, either shoot Merit to lower him and then shoot the twin to get him down. or shoot the twin and then shoot Merit. either will work, but if you are using Dart, be fast. because the next twin will start running to you, and if Merit is up then the shock shield will delay you. so by now one twin is down, and so is Merit. gun down the running twin until she is out, and then execute them. run back the the first twin you downed and execute them, and quickly find cover. you will get the achievement, but while you are running around like a madman downing two twins, and executing them, you also want to make sure there is some distance between them. which equates to time. so even though i covered the achievement process i also included to cover at the very end because Merit will be raising the elevator and gunning you down. the period of your last execution and Merit raising is brief, but i'd hate for you to put all this effort and Merit gunning you down while you aren't paying attention to him.