Halo Wars: Definitive Edition

This is easiest to do on the first mission--simply rush past avoidable firefights, and speed to Alpha Base. Once there, ram with your Warthog's and Grenade with your Marines and it'll be over quickly.
In case you'd like to try a different mission or just to see what they are, here are all the Par Times--note, you can finish the mission below the start of Par Time and it will still count!

Mission 1: Alpha Base: 5 to 12 minutes.
Mission 2: Relic Approach: 15 to 30 minutes.
Mission 3: Relic Interior: 8 to 18 minutes.
Mission 4: Arcadia City: 21 to 23 minutes.
Mission 5: Arcadia Outskirts: 21 to 35 minutes.
Mission 6: Dome of Light: 15 to 30 minutes.
Mission 7: Scarab: 12 to 24 minutes.
Mission 8: Anders' Signal: 18 to 30 minutes.
Mission 9: The Flood: 18 to 30 minutes.
Mission 10: Shield World: 23 to 32 minutes.
Mission 11: Cleansing: 20 to 30 minutes.
Mission 12: Repairs: 8 to 18 minutes.
Mission 13: Beachhead: 20 to 60 minutes.
Mission 14: Reactor: 15 to 30 minutes.
Mission 15: Escape: 20 to 33 minutes.

"Racing the Clock" Mission 1: Alpha Base: 5 to 12 minutes.
Seek in the video 3:30 Up until 15:00