Atlas, being the master of rock that he is, reaches under his feet and pulls up a stone wall – regardless of the material he may be standing on. It’s a magical feat, and one he doesn’t get enough credit for. This wall has its own base health that is affected by Atlas’ own Armor, so I recommend you mod for that (increasing Armor is highly recommended, for reasons that will become even clearer). Not only that, the wall is initially invulnerable for a short period, and any absorbed damage during this time is added to the base health. You can see the health of this wall at the bottom right of your screen as a percentage.
If you get bored with your little rock wall, you can recast to send it rolling off in the direction of your enemies, dealing Impact damage to anyone stupid enough to remain in its path. Eventually, it will explode, causing Puncture damage to anyone near enough to be hit. Interestingly, you can also cast your third Ability (Petrify) on the wall, which will cause it to move faster, further, and cause more damage when you send it rolling away – but it can only be cast on the wall form.