Dead Island Definitive Edition

Go to Bloodbath Arena DLC.
With this method you should get the achievement in less than 2 hours, go to arena C and fight until you reach wave 25+, at that point special zombies will appear carrying an exceptional weapon, after you kill him and get a weapon, just restart and repeat.
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The fastest way to do this is to go to Arena C and reach wave 25. This is when zombies will start to spawn carrying Exceptional weapons. I found 3 just in one game from waves 25-32.

You’ll want to be at least lvl 40 when you attempt this, and to play with a buddy as well so that you can revive each other. Reaching lvl 50 is an achievement anyway, so be patient and leave this for towards the end of the game.

I only write this guide because a lot of my time was wasted running around and opening chests after finishing the story, and that way is totally dependent on RNG.

To do this without using the arena, Fast Travel to Resort Bunker #02 and open the 3 chests there. There is 1 on the ground floor and 2 at the top of the ladder. You will need level 3 of Picklock to open all 3 chests. Once you have opened all of the chests, quit to the main menu. Then, select continue and open all of the chests again. With this method you can open around 9 chests a minute.

Exceptional weapons are orange and will either have a unique name or not have any adjectives before the weapon name.

To set your spawn location to Resort Bunker #02, accept the quest from Nikolai. You can get this quest back by navigating to:
Play > New Game > Character > Chapter 15 > Save file.
Then, head to the church and Fast Travel to Resort Bunker #02 and start the quest. You do not need to progress this quest in any way.