Back 4 Blood

Unfortunately you can only do this on veteran, you will need to ply through the whole of Act 1 up until the last level.
After about the 4th shell you’ve fired from the howitzer, an ogre will appear from the tunnel, if you’re quick enough you can shoot it there and then, otherwise you will have lure the ogre central in front of the howitzer shoot again. Luckily it only takes one shot and it doesn’t need to be bang on in front of the gun either.

Play all of Act 1 on Recruit up to The Sound of Thunder, the last chapter in Blue Dog Hollow campaign and also the last chapter of Act 1 overall. Make sure a character in your group has at least a combined 150% use speed from your active cards so you can load, fire and unload the howitzer very fast and kill the Ogre with a few shots before he drops down into the player area (The higher the use speed better of course). As a side note, these cards are generally also best used in conjunction with Doc and are always in the middle supply line in the menu.

List of relevant cards, their use speed and supply line:
- Headband Magnifier (+75% use speed) Bridge Town 3
- Multitool (+75%) Knuckle House 2
- Combat Medic (+50%) Furnace 2
- Screwdriver (+50%) Bridge Town 1
- Karlee's team bonus (+50%)

With the high use speed character loading, firing and unloading shells as fast as possible and the other 3 protecting them, you should have more than enough time to just blast the Ogre in the face with shells. Don't forget to put shells nearby before the first howitzer shot, although not directly behind the howitzer otherwise the backblast will send the shells flying away. Barbed wire also helps a lot.

The ratio is quite high for this achievement despite it not being that hard. I guess nobody runs a high use speed deck on their first go. The cards I listed in this may change at some point in the future and not be the most useful, but just keep in mind that you want as much use speed as you can get and you'll do fine. Also most of the guides say veteran because it had to be done on vet when it first came out.

This method/walkthrough is for anybody who hasn't got a full squad but it does rely on you having USE cards and decent STAMINA cards.
You can get this fairly easy by yourself as I have done. Yes, you cannot do this achievement offline BUT as long as you select online play even if you get put with bots (because the game is fairly dead at this point) it will still give you the 6 shots to close the mine at the end instead of the 4 it gives you in offline play, so the Ogre will spawn. To be honest the bots are fairly decent in this game on Veteran and I found it easier to play with bots than randoms.

Make sure you have at least 3 of the 4 USE cards below and use Karlee. Having good stamina cards is a must for the street and mines.

- USE cards
- Headband Magnifier (+75%)
- Multitool (+75%)
- Combat Medic (+50%)
- Screwdriver (+50%)

Make sure your stamina and use cards are first in your deck!
Play Act 1 on Veteran up to The Sound of Thunder, the last chapter in Blue Dog Hollow campaign. If you have already completed Act 1 then select your starting point on Act 1 as The Abandoned.
You don't want a Boss corruption so keep reloading your run until you get something different, ideally you don't want Snitches either unless you have extra offensive cards for grenades. I choose the M4 Carbine for my main and kept Karlee T9 for secondary, health pack, grenade.
First part/area is fairly easy, take your time and make it to the house for the hive. Best place I found for holding out whilst doing the hive is in the small room on the right on the 2nd floor with no windows. Hold out in here whilst killing the horde spawns, (3 in total) the bots will do most of the work and if you stand in far corner Tall Boys attack cannot hit you.
After the hive you have the tow truck and long street. Hit the switch for tow truck and stand right next to the car lifting up, as soon as you can go then GO! Haul ass straight down the street to the house at the end. If you get far enough from the bots when running down the street they will teleport to your position if they are all still mobile. Either hold out in the kitchen area (my choice) or the room upstairs with the small bathroom.
After it calms down, out the broken window, clearing enemies, over the log and onto the next open area with the first mine entrance. Clear all the enemies and blow the mine entrance. This is where stamina cards are a MUST because you're going to haul ass to the safe room. You DONT have to close/blow the mine entrances, so just run straight to safe room, don't worry about the bots or teammates, just get to the safe room and shut the door! I honestly found this part the easiest part of the mission as if you're quick enough you will out run 90% of the enemies. You might have to kill a couple of commons on your way but will out run the hordes providing you have good stamina.
Safe Room, heal up on the med cupboard, buy razor wire. If you're playing with a squad then all buy Razor wire. Head out and clear the area of enemies, load a shell ready and place 4 shells behind the cannon in front the wooden box (they won't despawn or fall under the map in this position) leave the shell in the box at the side of the cannon as this will be your 5th shot to take down the Ogre as it's right next to you. Place what wire you have around the cannon.
There is no 'timer' for when the Ogre will appear, he is set to spawn after the 4th cannon shot hits.
Fire shots 1 & 2, unload the cannon. At this point help your bots clear the horde, when ready load and fire shot 3/6, unload, load shot 4/6. Again help your bots clear the horde, take a breather, whatever, like I said Ogre won't spawn. Fire shot 4 (Ogre spawns) unload and load the cannon immediately with the 5th shot which will be right next to you in the box and fire straight away. If you're quick enough then you will hit Ogre and unlock the achievement. If you you're not quick enough and still have the continue then let yourself, squad, bots die, this way you will have another try from the safe room. If you miss both times then you can try and lure him in front of the cannon as per the other solution but I have tried and it is extremely difficult, you get overrun with horde after the 5th shot.

I have done this achievement twice now using the above method/walkthrough. Once for myself, solo with 3 bots which took around an 1-2 hours figuring out strats Etc. and the other time was with a friend and 2 bots. We got the achievement for my friend first try, took around 20 mins with me having the USE cards and running the MINE section.

I know the game has been out for some time now but thought I'd share my method for obtaining with bots.
Hope it helps!

I did this in offline mode with 3 bots on the easiest difficulty.

I did start from the abandoned start point in error but I did do this first time of trying

I used cards as described in other guides that recommend to use cards that increases your USE speed

Video below of my achievement but you don't see the achievement pop up due to Xbox capture

I was quite frustrating to do it solo directly choosing level 1-13, I have tried a few times, but the ogre quickly moves out and there´s no enough time to pulling out the used missile, inserting a new one and shooting. Playing with at least one partner was the best alternative for me (everyone gets the achievement, not only the player who fired the shot).

Here are the steps:

1 – Choose 1-13 The Sound of Thunder level (I suggest to be done on "recruit difficulty");

2 – You may use cards so that the grenade slots are bigger (player who will not shoot), but it is not necessary, as space for one or two grenades will be sufficient;

3 - If you notice that the offensive accessory in team upgrade is available, buy it to add one more grenade slot. These types are random every round. (If you didn't use cards, you'll have space for two of them);

4 – Buy as many flashbangs as possible.

You can razor wire the area where the Howitzer is, but it's not necessary.

Play normally until you shoot 2 projectiles, when shooting the third one the ogre will show up, so the player who has the grenades must be close to the spawn location and throw flashbang grenades in the ogre's range of action, he will remain immobile for a while. Keep throwing grenades to prevent the ogre from moving until the fourth shot is fired. Two players can manage it, but if you have more players, it will get even easier, as they will have to deal with the Ridden horde.

The advantage of this strategy is that you can select the phase directly and you don't have to worry about cards.