NASCAR 21: Ignition

Firstly it is worth saying that the multiplayer in this game is primative and frustrating. It is also quite unintuitive and prone to the bugs this game is plagued with.

There are no private lobbies, only 2 types of Public Matchmaking:
Shake & Bake (consisting of Super Speedway and Intermediate tracks, i.e. ovals above 1 mile long)
Bump & Run (consisting of Road Courses and Short Tracks, i.e. ovals 1 mile or less)
My experience is that Shake & Bake tends to be more popular easier to get into a race than Bump & Run (I'm playing during a Free Play Days event) but I also gather that normally the multiplayer is pretty dead so if you have a boosting partner, I'd recommend doing Bump & Run first.

Each option consists of a Matchmaking process that can take several minutes (and gives no info about what is happening) then a 1 minute countdown during which is a track vote between two named tracks and a third option of a random track and then load into the car pre race. You get 20 seconds or so to adjust settings (sometimes an extra minute if one or more players fail to connect) before the race abruptly starts. The race is 5% distance (takes around 8 minutes start to finish) with no Cautions or penalties (except that excessive short cuts can result in a lap not being counted). There is also a hidden end of race timer (90 seconds). At the end of the race the lobby is disbanded and you have to go through the matchmaking process again after EVERY RACE :(

Some extra tips:
1) Races are 5% distance, taking around 8 minutes each. No Cautions.
2) The full cycle of matchmaking and racing takes a total of between 11-15 minutes per race (assuming there are enough players to keep lobbies being setup). Therefore you're looking at 4-5 races per hour, or 20-25 hours to get 50 races in EACH type of playlist :(
3) It doesn't seem to matter for "completing races" how many laps down you are or even if you take the checkered flag at the end or time out. So long as you are in the race at the end it should count. Quitting a race midway through won't count (not sure about DNF due to terminal damage, is that even an option?)
4) There is no trackering in these achievements so try to keep a manual count of races and wins. For race I like to change the car number I race each time 1,2,3...etc.
5) You will quite often get random players trying to wreck everyone, just try to remain calm and not get too worked up. Remember you only need 10 wins and have to run 100 races, getting a 10% win percentage is not top difficult (especially if they wreck each other). Where possible try to race others cleanly as this where the most enjoyable racing can be had.
6) If you have a boosting partner it may be useful to coordinate when you both start matchmaking (e.g. via Xbox chat) in order to make it more likely you'd both be in the same group (still probably likely anyway). I'd also recommend boosting Bump & Run first, it seems harder to get a lobby of random players going in this than in Shake & Bake.

You need 100 races total, 50 Shake & Bake, 50 Bump & Run for these achievements. Hang on in there!