The method I used for this was pretty standard for all the DLC achievements:
Choose Strongholds map type, spawn Jerome, have him call in his Mech(you may have to wait for the T2 units at the 2 minute countdown but that's only 2 minutes), march over to near the enemy and let him blast away. He's a really fun kinda OP feeling unit early in the Stronghold match.
Other match settings:
Easy Enemy AI(I choose Cutter cause he's first....)
Put on any helpful skulls you want
Make sure to turn OFF Fog of War!
If you're going for other achievements then put the recharge power up to 200% and population per stronghold up to 10.
Time limit 20 minutes, just in case, if going for other achievements. You can always move in and kill the AI with a mass of units.
Don't forget to grab ALL the strongholds right away in the match, then spam build a huge army of cheap units and have them all gather near the enemy base. That should keep the AI occupied while you play with Jerome's Mech!