Halo Wars: Definitive Edition

Make sure to get the mandatory skirmish matches out of the way first, playing with all leaders, on all maps, etc. That will take at least 18 games, and will help a lot towards this and the other skirmish # achievements. Once you've done all the other Skirmish achievements, here's my recommended method for quickly getting the rest done!

Time to get 100 Skirmish Matches played:
Approximately 2 hours and 40 minutes.

General details:
After much testing, I've found that the Warthog Rush is faster than the Covenant Prophet Rush.
I have not done this at all Coop, but I expect Coop would be slightly faster as then you are only waiting for the loading screens and alternating Resigning.

Solo times:
The Warthog Rush VERY consistently takes about 93-95 seconds(including loading screens).
The Prophet Rush takes anywhere from 90-120+ seconds(including loading screens).

Loading screens are approximately 22 seconds total.

SKULL requirement:
The Warthog Rush does require the Pain Train skull(obtained in Mission 13 Beachhead)

Why does it work?
The Warthog Rush is consistently faster because you can always eliminate the enemy Warthog without it getting 'lost' on the map and wasting time.

How to do it:
Skirmish Match Setup:
Players: 1v1
Map: Chasms
Game Type: Deathmatch
Difficulty: Easy
AI: Any Human enemy--DO NOT choose Covenant! (They have much stronger bases)

How to do it:
When the match starts, immediately do these two things:
1. Select your base and queue up at least 4 Warthogs.
2. Press , select SKULLS, press 3 times, and activate the Pain Train SKULL.
Do the SKULL activation fast because it pauses the game while in the menu.
After that, select all units--, and start viewing to the enemy base while pressing to get your Warthog moving there.
Tell your Warthog to go about 3 Warthog lengths out from the enemy base's spawn point, and then doubleclick to make your global rally point there also!
Your Warthog will approach, shoot at the enemy Warthog--tell it to ram the enemy Warthog after it shoots once, then just sit and wait for the match to finish.

If the enemy Warthog doesn't wait for you...
In the rare case that the enemy Warthog decides to collect resources, follow it with your first Warthog(or any convenient Warthog) and blast it to smithereens. It will most likely go for the resources behind to the left and up from the enemy base. In very rare cases it might go into the middle or another direction, but I just watched and followed it.

Wrap-up:
That's all there is to it! It's consistent and faster than the Covenant Prophet Rush which I got annoyed with and experimented looking for something better.

I used the warthog Rush method but I had a minor adjustment that made it much quicker. Instead of doing Skirmish do online! Just set yourself up with an AI opponent, the reason it is faster is when you get done all of your settings are still saved and you can immediately start another match unlike with the Skirmish where you have to reset all of your settings plus skulls will still work!

There are two ways to get this achievement quick and easy, one is solo, and one is co-op.

EDIT: The times dont include load times or time setting up the match, only the in game match time.

Solo ~ 80 seconds per match

1v1 Deathmatch on Chasms
Playing as Regret vs. Easy Sgt. Forge
Immediately construct the Temple to spawn Regret, take him straight to the enemy base and use the cleanse beam on the warthog, the building being constructed in the back, then the main structure. Don't do anything with the ghost scout.

Sometimes the warthog will run to the back of the base for supplies, or a cyclops will be created and will sit in front of the base, so if the match isnt ending that is most likely why.

Co-op ~ 10 seconds per match

1v1 on any map, any gametype, and with any leader.
Have the person on the other team resign to get the win.

If you have a friend with the game who's willing to participate, you can get this done even easier. Set up a game between each other with any settings. The moment the game beings have your friend resign. You don't have to even play any of the game. Just rinse and repeat: ready up in lobby, loading screen, resign.