Full Umbral Atlas w PoS (1 Umbra Forma only!) build and explanation:

My build includes:

• [Steel Charge] maxed (Aura mod): you're gonna be spamming [Atlas]' number 1 the whole game; let's be real here. So to get increased damage for your Landslide, Steel Charge is strongly recommended to increase your Melee Damage.

• [Cunning Drift] (Rank 3) (Exilus mod slot): for [Atlas], range is also important as you want to maintain your combo for as long as possible so you want to hit enemies that are far away from you so I put Cunning Drift to increase Atlas' ability range a bit.

• [Umbral Vitality], [Umbral Intensify] and [Umbral Fiber]: these are a set for those who don't know: basically if you equip mods that are in a set in your arsenal (frame, rifle, melee, etc.), their stats will be boosted depending on how many set mods you put in your mod configurations. So by putting, all 3 Umbral mods on [Atlas], [Vitality], [Intensify] and Fiber get boosted values. My Umbral mods are all rank 8 as I'm broke rn but they will do just fine in my opinion. If you maxed out your Umbral mods, I recommend taking out Cunning Drift to get extra space, costing 10% of ability range which is not too much in my opinion.

• [Stretch]: This is mainly to increase [Atlas]' [Reach] with Landslide so he can hit enemies from a distance, also helps Petrify to turn more enemies into stone for Atlas to get Rubble.

• [Path Of Statues] Augment: This helps turn enemies into stone for free basically so you don't need to worry about using Petrify and you can focus on landing those tectonic punches while getting armor for free.

• Streamline: To help lessen the energy cost of Landslide to help you spam more.

• Transient Fortitude: To increase Atlas' ability strength a lot, sacrificing a bit of ability duration.

• [Continuity]: To help balance out the reduced ability duration from TF.

Arcanes:

I recommend using Arcane Energize and Arcane [Fury] as Energize gives a chance for you and your teammates to gain additional energy when you pick up an energy orb and for [Fury], gives you a chance to increase Melee Damage whenever you land a Critical Hit as you are bound to crit as long as you keep spamming Landslide so your overall damage will increase.

School:

I recommend Zenurik as Energizing Dash and Energy Pulse really help you get a lot of energy to spam Landslide.