Glad they at least announced it, annoying to have an online achievement in a single player game though.
And it's the most time consuming achievement in the game. It's not like the achievement in Saints Row IV that took 5 minutes to pop.
apparantly there is just a part of it that is online only, however you never know if you might lose the save somehow so better be safe and get it done.
338 000 and 15000th ach.
337 000, 3 000 this month
Halo 3: Complete Halo. - Halo: The Master Chief Collection
Collect 3,000 campaign or playlist medals. - Halo: The Master Chief Collection
Kill 1000 Flood. - Halo: The Master Chief Collection