Just like all the other DLC leader achievements, the Sgt. Johnson ones can be obtained pretty much the same easy way.If you want to do this the fastest method for just the 3 Wins, then do this:Setup a Skirmish Match with these settings:
Game Type: Deathmatch
Resources: Maxed
Fog of War: Off
Skull: AFTER PARTY(unlocked in mission 11, required for this method)
Any other helpful skulls can be turned on, put the AI on easy.
Don't forget to pick Johnson as your leader!
As soon as the match starts, build on all the closest build points to the AI base, then self-destruct the bases in order to get a tank and infantry unit each time from the Skull.
Repeat this until your population is maxed. (There is a 15 second timer before you can rebuild a stronghold point, so keep that in mind and try to time it so the AI doesn't snag any of your empty build points.)
Select all your units, move in, and wipe out the AI base! Use leader powers to help as necessary.
Otherwise, if you want to get all 3 Sgt. Johnson DLC achievements somewhat at the same time:*** Spoiler - click to reveal ***Here's I believe the fastest method to get all 3 of the Sgt. Johnson DLC achievements with minimum trouble. I played around with different settings and modes for a while and decided this worked the best to get all 3:
Set up a private skirmish game with these settings:
Map: Sentry(worked well)
Game type:
Strongholds
Time Limit: 30
Starting Population: 60
Add. Population/Stronghold: 10
Gameplay:
Unit power recharge times: 200%
Fog of War: Off
Leader Powers: On
Skulls!!: You will need to have these unlocked to be most efficient and it will help a lot!
Pain Train: 50% training for units
Emperor: Powers recharge 2x speed
Sickness: All units have 50% less hit points(I'm quite sure that affects the AI!)
Leaders:
Enemy AI: Easy--Atriox is who I tested it with
Pick Johnson as your leader 
1. As soon as the match starts, select your Armory and train Sgt. Johnson.
2. Then zoom around the map with the fast zoom and quickly build ALL the Strongholds around the map so the AI can't, starting with the ones closest to the AI.
3. Set a global rally point near the Stronghold closest to the AI base.
4. Start selecting Johnson and pressing 
every 30 seconds or faster to get 1 out of the 15 heal beams each time.(Do note, Johnson moves to where you press first, so make sure to activate it right on top of him so he doesn't move anywhere and waste time!)
5. Now you have a choice here; getting all 50 Colossus kills will take a while.
--- A. You just finish off the game after getting Johnson's heals.
--- B. You let the game continue until you get all 50 Colossus slams.
I'll assume you just go for all 50 Colossus kills first along with the Johnson heals, and win your other 2 matches using the Deathmatch method after.
6. Johnson's heal isn't going to be a problem, but you are going to have to wait for the unit levels to count down twice. The first time I believe it's 3 minutes before Tier 2 units are unlocked, then Tier 3 unlocks another 5 minutes after that.
7. Spend your time using Johnson's heal, dropping in the fortified infantry outposts and sentry towers near your stronghold closest to the AI, and spamming flamethrowers, snipers, and other infantry.
8. Wait for T3 units.
9. As soon as T3 units unlock, start building Colossus from that stronghold and the 2 other closest.
10. Send all your infantry to the enemy base to die, but keep Johnson to help heal Colossus.
11. Let your Colossus army build up, and take them in groups of 5 towards the enemy base. Press on any convenient enemy infantry and hope your Colossus gets a smash off in time!
12. It's very random, but if you keep sending groups of 5 to the enemy base, each time you should be able to get several kills. I believe individual unit models count as well, so a grunt squad would count as 5 kills!
13. Keep building up your Colossus army, sending in 5, healing at the stronghold with Johnson, and repeat until you get 50. It might take the 30 minute game, although in my 20 minute game, of which I had Colossus for only 12 minutes after they unlocked, I got my remaining 40 or so kills with the final one happening with 0 seconds left. LOL. I also had to figure out HOW to get the kills efficiently, because other units love to steal the kills before the Colossus can stamp.
You should get the Johnson achievement easily in your first match:
Your Colossus achievement in your first or second match:
For however many more wins you need, do quick skirmish games with the tank rush to get:
And that's it for the Johnson DLC! Enjoy!