Mostly putting this build here for myself, but feel free to use it.

Pros:

• Only needs one forma

• Super tanky

• Constantly regenerating energy from [Hunter Adrenaline]

• Good damage

• Decent range

• Flexible (Forma layout is friendly to most builds to allow for pretty good build diversity)

• Arcane Grace keeps your health mostly topped up, so most of your rubble will go toward keeping your 1500 armor topped up rather than getting eaten for health.

Cons:

• Requires umbral forma

• Not as strong as some specialized builds

• Slow casting speed on his Rumblers ability

• Relies (somewhat) on expensive Arcanes

Notes:
1) As an aside, (Umbral) Steel Fiber is a waste of a slot when Atlas Prime is already near-invincible with his passive, thus that slot is better off used for something else that gives a more immediate and impactful benefit.

2) You can get away with no forma on this build and all you really lose is the use of the exilus slot, and perhaps some build diversity.

3) If you really feel so inclined, you can switch the augment for more efficiency without really losing much of value.

4) The Path of Statues augment, at the time of writing (1/7/2020, patch 27.0.8), does NOT have a visual indicator for where the petrification path is. It used to show as a pile of rocks in a line, but now it's invisible. Hopefully we'll see a fix soon™