Sacred Citadel

Aside from trying to avoid the red X marks just prior to the rocks falling, you can also take advantage of the large mount in the previous screen. Getting hit by a rock while riding the mount will knock you off it, but will NOT damage your character, and the achievement appears to be linked to your character only,.The rocks only fall in one screen, but there are quite a few enemies in it. So consider using the mount if attempting this on your first go at the level. If over leveled, it is probably quicker to roll dodge and quickly finish enemies. Either way, good luck! As others have stated, it will not pop until completing the level.

The Citadel can be found in Act 4.

This achievement is made much simpler in co-op.

Your saving grace is the dodge roll (right stick) - it makes you completely invincible to almost everything in the game, including falling rocks (and boss attacks!).

The rocks in The Citadel level have insane AoE, so just roll until they stop falling. If you have a co-op partner, they can take care of all enemies so you can literally roll into the bottom right corner of the screen. This means you will also automatically roll off into safety once all enemies have been killed.

Easy peasy.