WATCH_DOGS

Here is a solution to get magic smoke AND Black hat trick. In Act 2 there will be a mission called Uninvited in which you have to prepare an Ambush for Iraq's men. BEFORE you hack the security terminal to start the countdown until they arrive, plant an IED (not a proximity IED a regular IED) on the tire tracks to the right of the computer (while facing it). The tire tracks will appear as a red square while looking at the ground. Once you plant the IED hide behind the shutter in a spot where you cant be seen, but can still press the button to trigger the IED.
Once the enemy vehicle is over the IED trigger focus and detonate the IED. This will net you the 2 achievements. If you somehow fail to get it just restart checkpoint. Much easier than doing it on criminal convoy.

*EDIT
Below is a great guide by Lil Miss Cherry that shows how to get the Magic Smoke and Black Hat Trick achievements.

The easiest way in my opinion to get this achievement is to start up a Criminal Convoy side mission. During these missions you will average about 3 to 5 cars that have at least 2 enemies each inside them. The point of these types of missions is to take down an enemy or two before they reach their destination and they always have more cars as backup. The reason why the achievement is the easiest during this is that all of the cars are extremely close to each other. All you have to do get them to stop by using environmental hazards such as blockers or just ramming the lead car, as this will cause them to bunch up together. The just throw an IED on one of the cars, and hold X over the bomb to detonate it. Depending on the type of Convoy mission it is, you may fail the mission, but the achievement will pop regardless as you are going to kill at least 6 people. The IED is available as a skill in the Crafted Items tree after you complete the "Open Your World" main mission and you must have already bought the Frag Grenade and Lure skills in order to purchase the skill. In order to craft the item, you must have an electronic part and a chemical component to craft it.

Step 1: Unlock the IED crafting in the Skill Tree.
Step 2: Craft IEDs using electronic parts and chemicals from the pawn shop.
Step 3: Go to a busy location, shoot some people and have the cops called on you. Once the cops arrive, they will drive in pairs, and park their cars near your position. Throw the IED between the cars, and explode it in order to explode both cars and kill 3+ cops to unlock the achievement.

Heyas, This is if your are fairly early into the campaign or had to restart to pick up achievements you missed!

Still in Act 1, the main story mission No The Pizza Guy.

After you have the Cut Scene mid mission you will exit the building your in, many bad guys that are 'low level' read dumb, will attack you. Using cover run from one side to the other getting all the bad guys onto one side with you on the other. Throw out IED's along any pathways they can charge on. They are not the smarts of back guys and run together in a group. Blow your charge and the cheevo is yours.

Note: This is recorded on the Xbox One, but the method should still work on the 360. Cheers!

Act 2 in the mission where you meet the hacker girl Clara, in your motel room. When you shoot your way out of there, alot of cars comes with 3 ppl in them, so plenty of chances to get it here.

Here is my solution for the "Black Hat Trick" and "Magic Smoke" achievements

You want to play online free roam with a friend and find a well populated area and then have your friend get the cops after them and the cops should be after your friend not you, so when the cops start piling you have the chance to walk behind them and drop IEDs or use focus and shoot them

Happy hunting guys

Video-guide of Watch Dogs to get the achievement Black Hat Trick. It consists in killling 3 enemies at once with an IED.

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Greets.