Mean and Green

Both Twisted Guardian and Enthralled Mutant in the Everbloom Wilds make a puddle of green slime for about 5 seconds after they die which you can walk over to gain a Mutagen debuff. There are also multiple Iron Pyrotech corpses scattered in the zone that have a green pool under them which you can also use to increase your debuff stacks. Every new application does increasingly more damage to you so be cautious about your HP when walking over a new pool. Walking over the pools stealthed does damage to you but it doesn't break stealth. The 1st stack only lasts for 15 seconds but gaining a 2nd stack increases the duration to 30 seconds and every new application keeps increasing the duration of the debuff.

To gain the achievement you only need 10 stacks of the debuff but you can get up to 20 stacks maximum. Leaving the Everbloom Wilds zone removes the debuff.

edit: the buff only stacks to 10 now

The easiest way to get this achievement is by killing the [npc=88394] at the east side of the Everbloom Wilds at around 73.90,39.45.

The npcs have less than 100k health, easy to kill with the debuff stacked.

Just FYI, the axebeaks that you hit with rocks will also create a pool when they hit the ground. However, you have to hit the flying ones. The ones in the trees will not fall down