Probably the easiest game to get the par times in seeing as they're very lenient times, the game is overall the easiest and the missions are quite short so doing them all in a few hours is quite feasible.
If you're doing Nagato Makes Moving Easy:
then finish that first and come back to scoop up the par times on the ones you didn't get as part of this, so you don't do multiple redundant run-throughs.
Simply load up each level on Easy difficulty (can be done on any difficulty but Easy will make this a breeze) and finish under the designated par times (turn on Competitive Timing in the settings before starting the mission to see your time as you play), which are listed under.
Tayari Plaza: 03m00s
Uplift Reserve: 04m00s
Kizingo Boulevard: 09m00s
ONI Alpha Site: 13m00s
NMPD HQ: 10m00s
Kikowani Station: 10m00s
Data Hive: 16m00s
Coastal Highway: 25m00s
Mombasa Streets and Epilogue do not count and can be skipped. If you need help finding a good route to take there are plenty of Par Time achievement guides, I recommend watching Halo Completionist's playlist:
https://www.youtube.com/playlist?list=PL3bi5dIhs8VX6glPSqXBN...