DOUBLE UPDATE UPDATE: Blitz no longer has a nice 2v2 mode either now! Now the only for non-ranked is Brawl, 3v3 Blitz. Blitz used to have a 1v1 which was perfect for boosting or playing Blitz fast. You also have the problem of running into only the tryhards who have very high level cards, and are simply impossible to beat because their units have higher HP, armor, and attack than yours. Just keep giving it a shot, hopefully you'll end up with random teammates/opponents that are worse than you, or aren't playing and give you a free win!
Blitz is basically a game-mode in Halo Wars 2 where you have a deck of 12 cards, and you can call in various cards based upon how much energy you've collected.
Calling in a card drops that particular unit or power where you are looking on the map.
You can make your own decks of cards or use the starter sets for each commander.
In order to win Blitz currently, you'll need to get some artillery and use it to hold down capture points. On the map Proving Grounds, B and C can be held with artillery and support units camped between B and C, and a few units on B and C.
Unless of course you are up against a new player as well!
Try to get to the energy crates as soon as you can because they are very important to survive longer--they give you energy AND increase the rate you collect energy over time.
This can be boosted, if you search at the same time as your partner, you have a very good chance of getting into the same lobby.
If you would like to see my preferred decks, let me know and I'll try to get them listed here!