Hello everyone. Being that the Missions times and bonus actions are already posted here, I will instead focus on telling you how to obtain this achievement.
First of all in tier 1 mode if you die you will be forced to start the level over again from the start. Also in tier 1 you will not be able to obtain ammo from your team! Tier 1 mode also has a timer (par time) you must beat. Lastly it is said that tier 1 is just like playing on hard but with these few changes included. So if you played on hard you should have a basic idea of what you are getting into.
Now the trick to not dieing is using cover, crouching, and laying down when you are getting shot and red blood is around your screen. It's wayyyyyyyyy better to take cover for 15 seconds and live than to die and start over.
Another way to avoid death is to allow your allies to take point! Never never never take point unless the mission forces you to! I can't tell you how many times I died because I ran up ahead and took point only to be killed by a cooked frag or an rpg!
As far as ammo goes it pays off great to switch all weapons to single shot mode (click d pad to the right). This will not only make your shots more accurate but it will conserve ammo big time!
Some weapons such as machine guns don't have single shot.
Also switch out shotguns for anything else, the last thing you want is a shot gun. Close range combat is not going to work here! A basic ak-47 is better! Don't pick up rpg's unless mission specific! The best enemy weapon is the sniper rifle and then the pkm machine gun.
Also there will be many times during the game where the mission will not
proceed unless you move foward. You may notice that you will kill enemies and they just keep coming back and taking up positions in the same spot.... This is when you must kill them and push foward to advance the mission.
My basic strategy was to get my team moving, and follow behind them, kill any enemies and move up hiding behind rocks or buildings as I advanced. Again when your allies are moving stay behind them, when they are not moving try to advance when safe to do so to get your team pushed up.
As far as par times go, if you follow what I say and don't rush things too much or take time to line up headshots you will always be under par time.
This guide is not for beating leaderboard times! And if you want this achievement don't bother trying to do so. It's better to just kill the enemies fast then trying to spend 5 seconds getting a headshot just to take off 2 seconds.
I beat all tier 1 levels in under 4 hours or so.
Good luck and please feel free to ask me any questions at all or if your stuck in a spot ask and I should be able to suggest what to do.