Atomicrops

Ok, let me start off this guide with this. The method I used to get this achievement is tedious and kinda sucks.

Year: 1
Character: Lavender
Upgrades I Had Relating to Fertilizer: Companion Planting, Avian Placenta, Mysterious Flower, Fat Grub, and Fraidy Fert.

Fertilizer:

For my run, I visited every biome, except the level 2 biome for the Tundra area. While I was out on my run, I found the giant slug, and I got as many fertilizer mushrooms as I could to kill as may enemies as possible with that mushroom. I ended Winter with a little over 3000 Fertilizer. So during Nuclear Winter, I took and ran around the farm killing as many of the enemies that the boss spawns. While doing this be careful not to pick up any crops, animal products, or flowers as this will do damage to the boss and kill him probably before you are able to get the achievement. This took a bit of time, but I was able to get the achievement this way.

(Notes)

I noticed than when playing the dice gambling game, one of the rewards can be Fertilizer. So right before Nuclear winter try gambling to get as much Fertilizer as you can to help the grind process.

Edit: I got this achievement a while after the Feline Good update for consoles.

Fala galerinha consegui essa conquista no ano 1 estava tentando a conquista da batata porém não sei como perdi todas elas daí estava com 2000 fertilizantes e pensei em tentar, consegui mode dos besouros onde vc pode encostar neles e eles te dão fertilizantes, também peguei o mode dos ovos que te dão sementes e fertilizantes, o tratos ceifador quando está com muitos inimigos use ele vai te dar muitos fertilizantes e também consegui o mode da flor aquele verde que fica te dando fertilizantes todo dia, então peguem o máximo de modes para a última estação no inverno no clear certeza que vai dar certo.
Boa sorte.