Here is the full list of all 11 Star Tactics achievements.
Destroy 3 enemy ships with one of your ships in a single turn
Deal the final blow to an enemy Dreadnaught with your Scout
Enemy ship is destroyed by its own torpedo, rocket or mine
Win Mission 30 of the main campaign without using abilities
Complete Mission 17 or Mission 18 with 100% HP
Reach Grand Admiral rank in Skirmish mode
Destroy 500 enemy ships in Skirmish mode
Destroy 3 enemy ships by gas cloud in a single turn
Win 5 battles in a row in Skirmish mode without restarts
Make the last remaining enemy ship to crash into an asteroid
Collect all 90 stars in the main campaign