Redfall

as some say it's not possible to do it in one move, YES it's POSSIBLE all you need to do is the management scheme:

1 step: make the achievement you want, as soon as you release the achievement on the screen, immediately do step 2.

2 step: you press the xbox button thus opening the hub by going to game management and add-ons.

3 step: delete saved data FROM THE CONSOLE and entering the game again.

because the save in the cloud consists of having a slight delay, thus being at the beginning of the mission right when you accept it at the planning table.

for this achievement all you need to do is as soon as you go to the gate and have the woman's dialogue she will open the gate as soon as you enter just shoot and kill the enemies in front of you and will unlock the achievement.

Missable & Mission Specific

The mission "One of Us" has 3 different outcomes depending what actions you take. Thankfully you can get two of the achievements within one playthrough by dashboarding/force quitting the game. To maximise Gamerscore on your first playthrough start by doing either the "Bloody Initiate" or "Sepsis" walkthrough. As you place the blood into the bowl you'll unlock the achievement. Now forcibly close down the game before it has chance to save. Once you load back in, you'll be at the deliver the blood checkpoint again. Just return to the Cultist camp and start shooting to earn the "So Anyway I Started Blasting" achievement too.

The third achievement can be grabbed on a second playthrough or by joining a friends game by using their save.

Sepsis Walkthrough:
Enter the camp from the main western gate, entering from anywhere else will make them hostile. Don't shoot or loot anything or again you'll become hostile. Go to the Rangers Lodge to meet Erica. Then go to the Bladewell Sea Caves. At the Bloodbag, contaminate the blood with gasoline by interacting with the barrel behind him. The three cultists in the room will become hostile (this is OK), kill them and press the button to drain the tainted blood. Take the blood to Erica at the Crane to unlock the achievement.

So Anyway I Started Blasting Walkthrough:
Enter the camp from the main western gate. As you enter the camp, start shooting the cultists to unlock the achievement.

Bloody initiate Walkthrough:
Enter the camp from the main western gate. Don't shoot or loot anything or again you'll become hostile. Go to the Rangers Lodge to meet Erica. Then go to the Bladewell Sea Caves. At the Bloodbag press the button to drain the normal blood. Take the blood to Erica at the Crane to unlock the achievement.

One of us is the first quest unlocked in the middle tree in Burial Point. You need to approach the campgrounds using the main road on the map so that you approach the front gate and make the cultists friendly. As soon as the gate opens you can step inside and blast on of the 3 cultists on the right and you’ll pop the achievement right away. Quitting the game and reloading did not de aggro the cultists so it’s not possible to get both this and Bloody Initiate in the same playthrough.