The hard part for this achievement is actually making the Annihilator pistol itself. I will go through how you would upgrade the 'Alistair's Folly' into 'Alistair’s Annihilator'.*NOTE* While going for this achievement, you WILL unlock the 'Alchemical Opus' achievement!
Step 1 - Getting the Alistair's Folly
There is two ways to actually get it:
1 - Try your luck out on the mystery box
2 - Mini quest to get a free one
If you want to do the quest, what you'll need to do is find four symbols around the map all with it's own colour (RED/BLUE/GREEN/YELLOW), input them into the machine in the library and, if it's correct, when you input the code it'll open and you can pick it up. If you enter the wrong code, it'll mess up what you entered and all you need to do is re enter the code correctly.
Video reference for the 4 symbols as seeing it is better than me explaining it (by MrDalekJD
) : Step 2 - Upgrading the Alistair's Folly to Chaos Theory
You'll need to collect two parts, the easier one being in the Library, As you walk down the stairs in the library, take a sharp left and there will be bookcases. The wall in which the painting is on the left there will be three bookcases (the middle being sort of split) in which youll need to use the shield to bash the middle bookcase which should open a small room with the part on the table.
The second part requires you to kill a werewolf with silver bullets. To save me explaining on where all 6 parts are for the silver bullets I will just leave a video guide for you to follow (it's easier with a video reference anyway)/
Silver Bullets tutorial (by Glitch
Once you have the silver bullets on your selected gun, all you need to do is kill a werewolf with the silver bullets and it should drop a part in which you take it to the Greenhouse Labratories, place it in the big machine in the middle of the Greenhouse (you place it on the side facing the fast travel), wait 15 seconds and a part will come out of the side of the machine (left side of where you placed the initial part in).
Once you have both of them parts, go to the work bench on the right side of the fast travel in the green house labratories (there will only be two work benches in there) and build the chaos theory and if you have the Alistair Folly while you're building it, it should become the Chaos Theory.
Video reference (by Glitch
) : Step 3 - Upgrading the Chaos Theory to Alistair’s Annihilator
You;ll now need to collect 3 more parts in order to upgrade the Chaos Theory.
First part is in the graveyard, if you go to the perk machine, there will be lanterns around the lower set of stairs with some being lit and some being off. What you want to do is locate the lantern that is lit and is slightly 'redder' then the other lanterns and shoot it with the chaos theory, once you do so, another lantern will be lit with the same reddish glow so basically you repeat this 3 more times in which a bat will fly out of the last lantern if you did it correctly, shoot the bat and it'll drop the first part. Note if you shoot the wrong lantern, you will have to wait 90 seconds before you can restart the process.
Next part is fairly easy, all you need to do is keep killing vampires with the charged shot of the chaos theory (so it makes them float) and they'll drop an item called 'bite', collect 4 or 5 then go to the coffin in the middle of the graveyard as it will now be glowing, interact with it and a red vampire will pop out, kill it and it'll drop the next part. Just to note, if you let the red vampire bite you, you'll unlock the 'Mega Bite' achievement!
Final part (and most annoying) is in the forest. You'll see glowing blue mushrooms around the forest in which you have to kill a zombies with the charged shot directly above the mushrooms, if done correctly the zombie will start digging and have a chance to dig up the last part. If the zombies does not dig up the part find another patch of mushrooms and make another zombie dig with the charged shot, keep repeating until the zombie digs up the part.
Once you have all three parts, go back to the Greenhouse Labratories and you'll need to interact with the machine in the middle twice (so place a part in the machine, pick it up a few seconds later from the side and repeat).Once you have placed the second part into the machine, you should unlock the 'Alchemical Opus' achievement. Finally take all them pieces to the opposite work bench in the Greenhouse and build the Annihilator.
Video reference (by Dosant
) : Step 4 - Unlocking the achievement
Once you have the Alistair’s Annihilator, all you need to is have a big bunch of zombies, hold a charge of the pistol and shoot it when fully charged, if you kill 15 then you'll unlock the achievement, if not keep repeating until you do!
I hope this guide helps :p*NOTE* You can't unlock achievements using modifiers, so when going for achievements, play on classic!