Rocket League

There are 4 achievements linked to Season mode and luckily you can do them all with the following settings:
• Team Size - 1v1
• Difficulty - Rookie
• Season Length - 9 Weeks
• Playoffs Teams - 4 Teams

You will play a total of 11 matches, 2 play offs and 9 matches and during this time you will face all 9 teams, after the 9th match this achievement will unlock

Rookie difficulty should be easy enough to win every match anyway as with only one AI they are very stupid. The Ai being stupid means that most of the time you can hit the ball in from the kick off, the 3 times you're positioned almost straight onto the ball boost all the way until you're close to the ball and stop boosting, then keep driving to hit the ball over them and hopefully into the goal (boost too far and it'll hit the top) when you're on the side of the ball boost half way then stop boosting and try to hit the corner of your car on the side of the ball facing away from the goal, takes a bit of practice.
Here's a video of what I mean, using the car 'Ripper'
You will unlock to following on the way:

WARNING: This is a very glitchy achievement and many, myself included, had a hard time getting this to unlock. However, I do have a solution I adapted on from circulating advice and can confirm works for me, hopefully will for you too . It might not for others, so please don't just down vote if it doesn't work for you; it isn't foolproof, just a suggestion that works more often than not...

I personally won a season and got awarded the Champion achievement for WINNING a season but not the Rocketeer one for completing it. I also got Team Player one match late during the play-offs, not after the main season when it should unlock.

I read forums and saw some commens saying that for some people it worked after sitting down during one sitting, and I can confirm that (although tedious) it DOES work.

What I did:
- Sign out
- Hard Reset Xbox One (Hold Off Button, unplug when off, plug back in)
- Turn back on
- Sign back in
- Load up Rocket League
- Play through a new season in ONE sitting (i.e. not turning off console, changing profiles or closing down Rocket League app)
- Most important bit - I made sure I lost nearly all my games so I couldn't advance to the play-offs. By logic, it was more likely to pop up for completing the regular season if the regular season was all I could physically do. There is a very big chance this isn't neccesary, but I gave it a go and it did work. I read the above steps didn't work for people, but they all seemed to try and win as well.

Unlocked after this method.

Ironically, for my last achievement ,Battle of Heroes, I played through a 3rd season on my backup account to acquire one of the necessary cars, and all achievements unlocked when they were supposed to. Luck of the draw, I guess...