Here is the full list of all 75 Halo Wars: Definitive Edition achievements.
Completed Mission 13 on any difficulty.
Mission 13: Destroy 25 units with the Scarab.
Completed Mission 12 on any difficulty.
Completed Mission 11 on any difficulty.
Mission 11: Save all the Airlocks.
Completed Mission 10 on any difficulty.
Mission 10: Distrupt all Tractor Beams.
Completed Mission 9 on any difficulty.
Mission 9: Put every Colony in Hibernation Mode at the same time.
Completed Mission 8 on any difficulty.
Completed Mission 7 on any difficulty.
Completed Mission 6 on any difficulty.
Mission 6: Successfully protect every Rhino.
Completed Mission 5 on any difficulty.
Mission 4: Save Adam.
Mission 3: Kill at least 5 Covenant Units with the Bridge.
Completed Mission 4 on any difficulty.
Completed Mission 3 on any difficulty.
Get a Gold Medal on any mission.
Completed Mission 2 on any difficulty.
Collect your first Skull.
Completed Mission 1 on any difficulty.
Unlock a Timeline Event.
Complete the Campaign on Heroic Difficulty.
Complete the Campaign on Legendary Difficulty.
Improve your Score in any Campaign Mission.
Finish all missions under Par Time on Heroic difficulty.
Get a Gold Medal with all Debuff Skulls Active.
Get a Gold Medal on every Campaign Mission.
Obtain a lifetime campaign Score over 1,000,000.
Mission 1: Ram and kill 50 Grunts with Warthogs.
Mission 2: Destroy every Methane Tank.
Mission 5: Jack 6 Covenant Vehicles.
Mission 7: Do not destroy any Power Node.
Mission 8: Use Elephants to train 100 infantry.
Mission 12: Repair the Power Core in less than 4 minutes.
Mission 14: Don't claim an extra base.
Mission 15: Open the Doors in order.
Collect all Skulls.
Score the winning point in a game of Keepaway against Legendary AI's.
Complete any Campaign Mission in Co-op Mode.
Win a Skirmish Game with every Leader.
Win a game on every Skirmish Map.
Win a game in every Skirmish Game Mode.
Get 20,000 points in any Skirmish Game.
Win a Heroic Skirmish Game against the AI in under 10 minutes.
Win a 3v3 Skirmish game with 2 AI Allies.
Use one of the 6 Major Leader Powers to destroy an enemy's last unit.
Win a Skirmish Game with Dual Scarabs.
Get a winning High Score with 10 or less Squads against the Heroic AI.
Unlock 20 Timeline Events.
Unlock All Timeline Events.
100% Completion.
Play Halo Wars for at least 24 Total hours.
Win a Public game.
Win and have the highest score in a Public Game.
Win 10 Skirmish or Multiplayer Matches
Win 20 Skirmish or Multiplayer Matches
Win 30 Skirmish or Multiplayer Matches
Win 40 Skirmish or Multiplayer Matches.
Win 50 Skirmish or Multiplayer Matches.
Win and never trail in a Tug of War game.
Finish five missions under Par Time on Heroic difficulty.
Win 100 Skirmish or Multiplayer Matches.
Collect 8 Skulls.
Win 75 Skirmish or Multiplayer Matches.
Finish one mission under Par Time on Heroic difficulty.
Get a Scarab during a Reinforcement game.
Gain at Least 15 population from a single wave in a Reinforcement game.
Finish with 5000 net resources tributed to your teammates on Memorial Basin except in Deathmatch.
Hot drop 50 squads to your Covenant Leader in a single game on Barrens.
Completed Mission 14 on any difficulty.
Completed Mission 15 on any difficulty.
Kill 20 enemy squads with a continuous Covenant Leader Power on Memorial Basin.
Disrupt 5 active Leader Powers in a single game on Blood River.