Halo Wars: Definitive Edition

A more time-saving method I found: start a solo campaign mission and AFK for one hour or two, then go finish the level but save the game just before the mission ends; every time you load the save and let the mission ends as normal, the completion time will add towards the stats, so just load several times and you will get the achievement without having to AFK for that long time.
This method also works for the Epic Grinder achievement(Obtained a lifetime campaign Score over 1,000,000) as both of the stats are added when a game ends. Note that the time only counts in gameplay(campaign, skirmish, and multiplayer), and the time spent in the menu don't count.

This only includes time in missions or in skirmish/multiplayer matches. time spend in the menus is not added. You can AFK in the first mission and then complete it to get the time added to your total, if you dont feel like actually playing 24 hours.