Call stratagems before you need them
Stratagems have a call-in delay and a cooldown. Throwing one as you approach an objective means it lands when you arrive. Calling it as you start fighting means it lands after you have already lost people.
Watch the beacon
Every stratagem lands where the beacon is, and beacons bounce. Check where yours actually landed before running, and call out before you throw an offensive stratagem into a teammate's position.
Stick together, but not stacked
Splitting up means reinforcements arrive into bad situations; stacking means one orbital kills the squad. Move as a loose group within supporting distance.
Reinforce away from the threat
Throw reinforcements behind cover or away from the enemies that killed the squad, not onto the spot where they died.
Objectives before kills
The mission timer and objective completion decide rewards, not kill count. Clearing a patrol quietly and completing the objective beats a prolonged firefight.
Extraction is a mission phase
Plan for it: keep one stratagem off cooldown, clear the landing zone, and do not wander off for samples once the shuttle is called. Most mission failures happen at extraction.
Difficulty is a team decision
Step difficulty up when the squad completes objectives without losing the whole team, not when one player is carrying.