Dead Effect 2

This is the most grindy achievement in the game by far. It requires you to collect or purchase a ton of rare parts, and you have to make sure to get rank 4 in each of the weapon training categories. Sometimes you have to do a couple of extra prerequisites, like certain side tasks or finding certain items. These requirements are outlined in Steam LinkSavior8801's ultimate weapons list . I am going to be outlining a few important things when it comes to grinding this achievement.
Linksavior 8801's ultimate weapons list: https://steamcommunity.com/app/394760/discussions/0/3570700856103483370/

Firstly, you will need a ton of boss kills and I recommend having the DLC for this, as it will make grinding boss kills very quick. On the first DLC mission (in part 3 of the story) you can kill the boss at the beginning of the level, and then restart the mission and it will still count towards your progress. You do not need to complete the mission for the boss kills to be counted at the end either, you can just click on "return to base" in the options menu.

For some of the weapon training, you will need to farm soldiers. To do this I typically picked a "moar bulletz" generic mission, killed all the soldiers, then repeated until I hit the required amount.

Sometimes you may need "dead spacer" kills. These are the zombies that you will see wearing the space suits as you play the game. If you need to farm these go to the second mission in story part 1. Quickly grab the key card at the beginning of the level, use the elevator, then go through that section and kill the dead spacer that's in the room after the hallway. You do not need to finish the mission for it to be counted, you can just click restart the mission or back to base in the pause menu and it will still count.

Cannibal kills can be farmed in the 4th wave of any biohazard mission. Make the mission longer in the menu before starting if you need a lot of kills.

Ninjas can be farmed in a DLC mission from story part 3. I am pretty sure the mission is called "Holes in Space and Time". There are over 10 ninjas at the beginning of this level. Kill them and then restart the level until you have the required amount, then back out of the level. Your progress will still be tracked.

Generic zombie kills can be farmed in biohazard or survival. I recommend doing biohazard because zombies spawn more quickly than they do in the beginning phases of survival. Also, the game has trouble will longer survival missions for some reason.

CREDITS are the most important part of this grind, and there is a super effective way of grinding them. Each rare part costs about $1,000,000 credits, and each ultimate weapon will require lots of these parts in order to craft them. In the generic mission, Jetlift Cargo Area - Seek and Scavenge, you can earn around 800,000 credits for each run through of the mission. Do this on hell difficulty. The mission takes about 1 minute to complete. To do this efficiently you need to find all of the credits in the level. They are all in obvious locations, and you should memorize a route for grabbing all of them quickly. You do not have to kill any enemies to complete the mission. Full completion of this mission is required to get the full amount of credits.

Hope this helps! 😃

I found that grenades did not seem to work for this, so i ended up using a grenade launcher to kill the spider instead. This will work on any difficulty, so choosing the easiest can make it go by alot faster.