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