Rampage

Unlock the tower knowledge Blessing of the Ancients to get blessings in Torghast. Blessings and Torments are fixed each day for a particular wing of Torghast and reset when daily quests do. So if you want this achievement, you'll need to keep checking back every day for when the Blessing "Rampage" is active.

Once "Rampage" is active, this achievement is very easy to do. Just kill enemies to stack the buff up to 10 and then stagger your kills of further enemies to refresh the buff for 30 or more seconds. You only need 1-2 refreshes for it to last long enough for the requirement as the buff lasts 16 seconds on its own. Small enemies like mawrats and skeletal remains give stacks like any others do, so if you are undergeared and finding this challenging for whatever reason, you can just find a pack of those to camp for 30 seconds.

You get the achievement upon completing the floor. You do not still need to have Rampage active at the end.

You must have 10 stacks of the Rampage buff. You need to hold it for 30 seconds. Once you've held it for 30 seconds total you have to let the buff fall off and you will get the achievement. Continuing to hold the buff won't grant the achievement.

Looks like at least Blizz is aware of the issue with not getting credit for this achievement. Not sure when they'll fix it though.

https://us.battle.net/support/en/article/358494