Two things.
First: Bonus Objectives, the ones you need to complete all of in order to get an Achievement.
Do you have to do each level’s bonus objectives all in one go, or can you play through the level once, complete some of them, and come back & play it again and complete the others? Also, is there any way to track which ones you have and haven’t completed (as in [pulling random ones out of my @$$ here], “Let’s see, according to this I still need to get 10 Light-bridge kills on “Hold the Line”, and I still need the ‘Destroy 15 Blisterbacks’ and ‘Kill 40 squads’ ones on ‘The Foundry’…”)
Second: Skulls, and Achievements
In previous Halo games, turning certain skulls on invalidated Achievements, and in the only other RTS I’m familiar with - StarCraft II - using any cheats also perma-disabled Achievements for that playthrough.
In Halo Wars 2, if I turn on any of the beneficial Skulls (Pain Train, Bountiful Harvest, Emperor, Sugar Cookies), will I be unable to get any Achievements? Will I be unable to get SOME achievements but still can get others (namely, the ones for completing all bonus objectives and… I believe I’m still missing the “kill the Brute Warlords AND all of their minions” Skull in the last level [I must’ve missed a minion or two])?