After June 1st go into a mission with just Rookies and beat the mission.
Some tips to help out:
Try to have at least 5 squad members if not 6 (I used 5).
Grenades are your friend, especially if 2 or more enemies are bunched up.
Try to limit engagements to one group of enemies at a time.
Save and reload. If you know where the enemy will be you can better prepare for each group of enemies.