Saints Row 2

This achievement is awarded for the completion of all 5 Hitman activity locations.

This activity DOES NOT need to be done in co-op for the co-op activity achievement.

The Hitman activities are in the following locations from north to south:
Hotels and Marina District
Trailer Park District
Stilwater Prison District
Saint's Row District
Barrio District

On arrival at a location, you will be presented with a list of all the people you will need to take out. Just select one and it will be added to your HUD with a possible location on the map to find the hit.
Technically the hit can spawn in other regions but it is most likely to show up in the district listed in the hit.
Now comes the boring bit, you'll have to search for the hit until it shows up on your mini map, most hits have conditions you have to satisfy to make the hit appear (dial a number on your phone, drink some booze, cause police notoriety etc.) but I have mentionned the requirements for each below.
When the hit is nearby, a crosshair icon will show up on the mini-map. You can then use the bigger map to get the exact location. Once you find the target, it doesn't matter how you kill them as long as they die. Be careful though, if you get too far away from the hit then the crosshair icon will disappear and you will have to find it again.
After each hit is completed, you'll need to return to the activity location to activate a new hit on your HUD

There are 5 locations each with 6 hits each so that's 30 in all to get this achievement.

Tips: Make sure you save after each hit, because the game does not autosave during this activity. There's nothing worse than having the game freeze after you've done a good amount of this, and we all know how bad this game is when it comes to freezing.

It may be easier to do this at the end of the game, ie. when the gangs are no longer around as they can be an unwanted distraction. Also avoid using homies because if you start shooting a hit, they will help you kill it and it they get the killing shot then it will not count.

This activity DOES NOT need to be done in co-op for the co-op activity achievement. It is better to avoid doing this in co-op aswell as it could confuse the game if different people are killing different hits. It is better if you physically kill all the hits yourself.

Note: these are only locations where I found my hits, there is no guarantee that this will be exactly the same for you.

Hotels and Marinas District Hitman:

Jeremiah - Marina/Museum Districts: Found this one in the Museum District, he's riding a motorcycle.
Nate - Marina; Poseidon Casino: Played a poker machine in the casino nearby for a bit like suggested by the hit description and Nate popped up after I finished playing on the machine nearby.
Brian - Anywhere: Easy one, just dial 911 on your cellphone and he will drive up to you in an Ambulance, just make sure you're near a road.
Chris - Museum Gift Shop: In the Museum District, towards the top right corner. There's an ancient ruins type thing on top of a cliff. The shop is south west of this and it's marked as a Clothing store called Buy Jove on the map, Chris should be waiting outside.
Randy - Marina District: This one spawned for me in the Marina District just up the steps from the this Hitman activity. I needed to be driving a boat for Randy to appear. This one was a decoy though, the real one appeared after me after I killed the decoy.
Nick - Marina; Heron Hotel: Marina District again, the Heron Hotel is the next tall building along from the casino. Just go inside and take the elevator up and Nick shows up in one of the rooms.

Trailer Park District Hitman:

Shannon - Trailer Park District: This one spawned in the Trailer Park's Booze store/Laudromat building. I had to get into a fist fight with the locals for him to spawn.
Jim - Nuclear District: Simply go to the Nuclear District (south west island) and dial 555-oops (555-6677) and Jim will show up in a Septic truck.
Roje - Suburbs Music Store: Go to the Scratch That music store just outside the Trailer Park, on the edge of the Suburbs. He didn't spawn until after I purchased some music there.
Mike - Downtown District: Do as the description suggests and get SMOKED to get teleported to the hospital. He spawned inside the hospital as soon as I respawned at the hospital.
Clint - University District: This one spawns in the University District, I found him on the west side. This one is a decoy though. As soon as you kill the decoy, the real hit will spawn and he's in a car so you need to be quick or he may disappear.
Greg - Freckle Bitch's: The nearest Freckle Bitch's is off the north east corner of the Trailer Park District. Just buy one of every item on the menu and Greg will spawn nearby. He has 2 bodyguards but there is also a rival hit man trying to steal the hit so kill Greg quickly and don't worry about the bodyguards.

Prison District Hitman:

Everett - Red Light District: Not too difficult as long as you look around the Red Light District. You're looking for someone dressed as a sailor. You don't necessarily have to kill a pimp like the descroption says.
Justin - Suburbs Cemetery: The Cemetery is located north of the Trailer Park District in the Suburbs. Description says between 10pm and 4am but I managed to find him at 2pm so not entirely true.
Chris - Arena/Project Districts: Found this one in Project District on the most northerly road which leads to the Arena District. You're looking for a biker type guy on a motorcycle.
Tim - Anywhere: An easy one, just dial 555-FIRE (555-3473) and Tim will appear in a Fire Truck, as long as you are on a road.
Mitri - Factories District: Found this one in the Factories District, he spawned directly under the Freeway Bridge on the east side of the district (near the Spelunkers clothing store). Just drink a 40oz liquor drink and he shows up.
Frank - Anywhere: For this one, you need to get a 5 star police notoriety so the FBI trucks show up to eliminate you. Frank should spawn in the first FBI truck after getting 5 stars.

Saints Row District Hitman:

Apoop - Barrio; On Track: There is only one Night Club in the Barrio District, just look for the liquor store marked as a Night Club in the Barrio District (it's next to Sloppy Seconds) called On Track (where you fought Veteran Child of the Sons of Samedi). The hit can be found anywhere in this place, it has 3 floors.
Larry - Trailer Park District: Just keep running around the Trailer Park District and Larry will emerge from one of the trailers ("Just to note Larry in the trailer park did not spawn for me until after I equipped the undershirt mentioned on the hit checklist. I ran around for awhile in there before buying one. As soon as I had it on he popped right away." thanks to Lonesquiff for mentioning this).
Seabaugh - Marina; Pirate Ship: For those who don't know where the Pirate Ship is, it's on the north side of the Hotels and Marina District near the Hitman location for that region. It's easy enough to see from there. I encountered a rival hitman on this one, so kill this one quickly.
Mr Flegel - Red Light District; Rusty's Needle: Go to Rusty's Needle in the Red Light District and buy a tattoo (doesn't matter what), after leaving the parlor, Mr Flegel spawned inside getting a tattoo done.
Lt Freeball - Anywhere: You'll need to get police notoriety, Lt Freeball showed up at 3 stars for me. He's the one who's not wearing pants!
Russell - Underground Caverns: To get to the caverns, you need to find the building on the east side of the Factories District with Phantom Caverns written in big letters on it. You have been here on numerous missions. The caverns are quite big but luckily the hit spawns in such a wide area on here and it's hard to lose. I also encountered a rival hitman on this.

Barrio District Hitman:

Alvan - Arena District; Drive In: For this one, I needed a couple of 40oz's which you can buy from Booze stores. The drive in is in the north west corner of the Arena District, it's up over a bridge leading over the canal. Once you are there, drink your 40oz's one by one until Alvan appears. He appeared on top of the drive in building.
Brad - Barrio District: You'll need to spray a tag in the Barrio District for this one. There's is one just south of the Heli Assault in the Barrio District. Brad will then appear. (EDIT: "he will still show up if you've already done the tags. Select him and stay in the area and eventually he will spawn (at least that's what happened for me)" - Thanks to fronkdegronk for this)
Anoop - Anywhere: This one will appear when you've got enough police notoriety. Took me 3 stars to get Anoop to appear, wearing an exotic shirt instead of normal police uniform.
Frank - Anywhere: Another easy one, just dial 555-455-8008 and Frank will show up in a taxi.
Scott - Anywhere: Easy one again, dial 555-6328 for Freckle Bitch delivery and the delivery driver should be Scott.
James - Suburbs; Technically Legal: Technically Legal is located just north of On the Rag (not the one North of the Trailer Park but East of the Trailer Park). There's a big red sign on the south side of the building. When inside, do a ho-ing diversion, I only did Level 1. James then appears, but this one was a decoy. I then had to kill the real one.

Please leave a comment if there is any way I could improve this solution, as sometimes slightly different things can happen to get people to spawn. I'll add it in and give you credit. Thanks :)