Rocket League

Edit 3: This now appears to have been patched and is working fine

Edit 2: This is possible to obtain if you have the retail disc. See Dimmocks solution for more details.

Edit: This appears to be unobtainable since a recent update. Nobody only TA has unlocked this for a while. I will update the solution if there appears to be a fix or if this is patched.

To unlock the Armadillo vehicle you need to win a season championship, the quickest way being against 9 teams on 1v1 and on the Rookie difficulty.
To unlock the Hogsticker you simply need to win a match in every other car, the quickest way being an exhibition match against no bots and changing the mutator to a max score of only 1 goal

Now you just need to win a match using one of these vehicles against someone using the other one. There are 3 methods of obtaining this:

Firstly you can use a second account to try and unlock one of these vehicles, the easiest being the Hogsticker. Using the exhibition method described above you can unlock and get a win in every vehicle in a fairly short amount of time. Once you unlock the Hogsticker, start an exhibition match using 2 controllers on both your accounts and win the match with your main account while using the Armadillo.

Secondly you can simply boost this with a friend in a private match over Xbox live assuming you have both unlocked a seperate vehicle

Finally you can just attempt this in a random online game. As the Hogsticker is easier to unlock and therefore more likely for an opponent to be using it i suggest playing a bunch of 4v4 games with the Armadillo.

Edit - As of 23/3/2017 this achievement is obtainable online.

I can confirm that if you have a disk copy install it offline so you have no updates and stay offline while you do it so the achievement will unlock when you reconnect to xbox live.

1. Uninstall Rocket league and delete save data(console side only).
2. Install Disk copy offline and do not reconnect to xbl.
3. Complete season mode offline, 11 matches to unlock the Armadillo.
4. Go into exhibition mode set the goal limit to 1 goal and using a guest+second pad have that guest win a match with each car until you have unlocked the hogstiker.
5. Defeat hogsticker with your main accounts armadillo.
6. Connect to XBL and the achievement will unlock. I connected on the results screen after I won because I was impatient.
7. If you have not already deleted the new save data off your system make sure to select your cloud save when you launch the game again so you do not lose your main saves progress.

The hardest part was waiting for the RNG to give the guest account each car.

You can get it in the way mentioned above, but it is possible to get with going online or making a separate account. All you will need is another controller, and it will take around 30 minutes.

At the main game menu press the join button on the second controller. Select "add guest" and a new car will pop up next to yours. Go into "expedition mode." Press "mutator settings" and change the goals to 1 goal, so the match will end when 1 goal is scored.

Exit the mutator settings and click "create match." Now you will need to score a goal with either controller. (preferably yours, because you will level up faster if you score the goals) Once the match ends collect the items and press "play next match." You will need to repeat this until the guest account has every car in the game that it can win. You can see this in the garage, you will have all cars except 2, will be 2 empty slots.

When the guest account has won every car that it can win, you will need to win 1 match with everyone of those cars. The easiest way is to select the first car, go to "expedition mode", score a goal with the guest controller, winning the match. You will then need to go back to the garage, select the second car, and win a match with that car.

Once you win a round with the last car, you will get the HogSticker. With the main controller choose the Armadillo in the garage, (this is achieved by winning a season) and win a match with it. The achievement will then pop up.

Note: Do not exit the game or remove the guest, as you will loose all of the progress from it.

This achievement is officially fixed as of 3/22/17 with the Dropshot update! I can confirm I've been trying to obtain it for months now but after the update I got the dropshot achievements and went for this again in split screen and got it! Rocket League is officially 100% :)

So there are a lot of guide that mentionned you need to unlock the car using another account.
But there is a simple way to do so

If you have already unlock both of the cars you just need to turn on a second controller and just add a guest. Then join your group on Rocket League (with the button.
Go to the personalisation menu and change the car of each account.

Now, you just have to go on an Exhibition match, set the mutator to "1 goal", launch the game and score.

Here you go !

So far none of the methods work. I've tried to get it by everything I could think of: Getting split screen online, exhibition split screen, guest account, everything. My only hope seems to be just playing random games, hoping that someone else has the opposite car than me. Anyone else notice this issue?

here is 3 possible solutions for this acheivement use a hogsticker or armidillo and hope that the other online guy has the opposite of yours, 2 go in a private match with your friend and unlock it that way, 3 go in a splitscreen match and do it that way.