Green (push-type) cards have low attack, high health, armor to reduce damage, and will push back offensive units. Place them in your front lines to protect your gunners. Eventually, place them in the rear of your lane to prepare another offensive wave.
Use push-type cards to defend low-priority lanes. Using a single high health push-type card in non-essential lanes can slow your enemy's progress long enough to let your main lane to win.
Certain Purple (tactics-type) cards inflict direct damage to enemies in a lane, except against armored units. Only use them when you are sure you can get a kill. This prevents your opponent from building up their forces and gives will you more control over a lane without risking troops.
Certain tactics cards boost your troops' damage output. Use them on Push-type cards or on any of your cards when you have units next to your enemy's headquarters.