Three Car Garage (25 player)

to do the chopper part solo, just pull Levi with Demolisher, then quickly go back through where the yellow gate will appear, stay close and wait for him to agro the Demos behind the gate, then throw 5-6 stacks of the DoT (2) ability on the boss stuck in fence, mount the chopper and wait for it :)

If you're trying to solo this achievement with the Siege Engine, just hop into the passenger gunner seat and blast FL with the cannon. I can confirm you do not need to be in the driver's seat to get Siege Engine credit.

This achievement is definitely bugged. I defeated Flame Leviathan several times while in a Salvaged Siege Engine (that is the last criteria I need) and I have not gotten the achievement. I opened a ticket and this is the response I received:

"Thank you very much for reporting this issue you experienced with the <Three Car Garage> Achievement. I'm adding your information to the report to help with the investigation on this issue so that we can get it addressed by our Development team. While the Game Master department greatly appreciates your assistance in addressing achievement issues, unfortunately the Development team does not currently allow us to alter or edit most achievements due to the setup of the achievement system. Because of this setup, the Development team only permits the Game Master department the power to alter a few specific achievements that have verified issues we are addressing. Because this isn't one of those yet, we aren't going to be able to directly act on this achievement for you. I'm terribly sorry for the inconvenience this can cause. "

So essentially the GM admitted this is bugged but the GM department cannot start awarding these as the Development Team has not looked into yet. I recommend for everyone to report this bug because unless you really bring something into Blizzard's attention, they will not fix it.

This seems to have finally been fixed.