I found the achievement requirements to be a little confusing, so to clarify, you must perform THREE assassinations: 1) Cassidy in the vault of the Schimdt house with the laser security system, 2) Cassidy while his house is being fumigated, and 3) Janus with an explosive packed in a mole hole. These executions are outcomes of the "House for Sale" (Cassidy), "Pest Control" (Cassidy) and "Whack-A-Mole" (Janus) mission stories for Whittleton Creek. If you want to have the game guide you through them:
1) follow the "House For Sale" story. When Cassidy is in the house, take him down to the basement and show him the vault. Disarm the security system, let him go inside to check it out, then rearm it.
2) follow the "Pest Control" story, then go inside the house and kill Cassidy while the sleep gas is pumping. You have to wear the exterminator disguse to avoid the effects of the gas. Any method of killing Cassidy should work. I used a basic neck snap. (EDIT: Some people have reported in the comments that a neck snap didn't work for them, while others write that it did. One player used lethal posion in the fumigator. If the achievement doesn't unlock for you after a neck snap, reload and try something else
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3) follow the "Whack-A-Mole" story, wait for Janus to come outside and inspect your work, then detonate the explosive when he is near it. You want to be away from people (not at the Wilsons' party!) when you detonate, because chaos will ensue, and everyone will start to suspect you pretty quickly no matter how innocent you think you look.
Note that the second and third executions are messy and noisy, so don't do them when you're trying to get a Silent Assassin rating.
If you insist on not following the Mission Stories but still want to unlock this achievement, you can find the realtor at the muffin stand in the NE section of the block. Acquire his disguise and go speak with Cassidy. Get the sleep gas from the green storage shed in the construction area and use it in the fumigator connected to Cassidy's house when Cassidy is inside. You MUST have an exterminator disguise to enter the house when the fumigator is running. To get the remote explosive, listen to the deputy and the gardener argue near the road work area. The deputy will confiscate the gardener's remote explosive and store it in the open chest next to the sheriff's vehicle in the road work area. Go to Janus's property as a gardener with the explosive and a bunch of other stuff to cram down the mole holes. There's one specific Janus bodyguard in the backyard that you talk when you're ready for Janus to come check your work.