Nightmare Ender, Nightmare Euthanizer and Nightmare Terminator
There is a separate guide for these that can be found by clicking here.
Aftermath Arachnicide and Aftermath Cobweb Cleaner
You will get Aftermath Arachnicide achevement for simply killing the Spider Queen one time. Once you do, the Aftermath Cobweb Cleaner achievement will unlock. To get that, you must kill 5 cacoons. Throughout the fight, players get wrapped up into cacoons. Get the finishing blow on 5 of these (doesn’t have to be during 1 event) and you’ll get the achievement.
Aftermath Vermicide and Aftermath Shock Wave Jumper
You will get Aftermath Vermicide achievement for simply killing the Toxic Wurm Queen one time. Once you do, the Aftermath Shock Wave Jumper achievement will unlock. To get that, you must jump over 5 shock-waves as seen below. Throughout the fight, the Toxic Wurm Queen will spit out shock waves on the floor. Jump over the tip of the shock-wave to get credit for jumping over one. I found that the best way to do this is to engage the boss and just stand there shooting something long distance, jumping over them until more people show up. You can either camp the spawn to do this and get it guaranteed quickly, or try your look while following the zerg. Try to group up to raise the chances of it shooting at where you are standing. Jump over them 5 times and you’ll get the achievement.
Aftermath Toxic Counteragent and Aftermath Detoxifier
You will get Aftermath Toxic Counteragent for simply killing the Toxic Alchemist one time. Once you do, the Aftermath Detoxifier achievement will unlock. To get that, you must kill 5 toxifiers. The toxic Alchemist has 2 phases. One where he’s a Veteran, and one where he’s a champion. During the Veteran form, he will periodically disappear and toxifiers will spawn in the play area. Kill them to count towards the 5 toxifiers you need for the achievement.
Aftermath Treasure Hunter
You will get Aftermath Treasure Hunter achievement for simply using the Tricolor keys on the chest 5 times. If you have a lot of Pristine Toxic spores, that’s the way to buy keys from Majory’s Helper Dee in order to acquire keys. You will need 45 silver and 30 Pristine Toxic Spore samples per Tricolor key.
To get the Aftermath Treasure Hunter achievement, you need to complete 10 events around Viathan Lake in Kessex Hills relating to the Nightmare is Over World Event. This involves killing the Spider Queen, Toxic Wurm Queen, and Toxic Alchemist a total of 10 times.