This is a remarkably quick way to get this done! You need a boosting partner, and it's recommended you go for this achievement at the same time:
The Space Healer achievement in Halo Wars 2 worth 90 pointsCast 100 Healing Leader Powers (Multiplayer, Skirmish or Blitz)
Anders is a good choice for a leader power healing card--She has one that costs 70 energy.
1. Find a boosting buddy
2. Make a deck that includes all cheap cards and possibly a healing power card(Anders is a good choice if you're going for the healing powers achievement at the same time).
3. Setup a Blitz game with your buddy, and start spamming all your cheap cards!
4. Send your units into kill zones of the other person, and spam spam spam cards.
Throw in a leader heal power now and then if you have it....
5. Make sure to share the energy drops!
6. You get 50% energy back from killed units, so you can really spam away sometimes when you lose your units!
7. Try to call your units in right near the enemy kill zones, because when they spawn outside of the home base they have 50% health for 8 seconds and are easy to kill.
I was able to get 68 cards in one game this way, and I think 12 leader heals? And that was with some goofing off and trying to get counter kills, etc. This could be optimized a lot more than we did.
It goes really fast, way faster than anything else I've found.