7. Leveling your Support soldiers
These units not only provide a ton of utility, but are also your secondary alien killers. Sprinter is practically mandatory, as it'll help you get to low-HP units in time to save them. If you're running with two Supports, I'd suggest making one of them your dedicated healer, picking up the Field Medic, Revive, and Savior abilities. The other should keep your units protected--grab Smoke and Mirrors, Rifle Suppression, Dense Smoke, and Sentinel.
Recommended loadout: Plasma Rrifles, Medkits (don't forget to upgrade Medkits via the Foundry)