This is my idea for Umbral [Atlas Prime] without using Umbral Forma on him. I added the augment for our most useful skill "Landslide" which is [Path Of Statues], Path Duration is more than enough (10.85s) and Petrify Duration is more than you will ever need (21.70s). Our Boxer must be tough, so I gave him [Adaptation] and Arcane [Guardian], but you can replace this Arcane with whatever you want. For example Energize or Grace.
New amazing change here is augment [Rubble Heap] which let you use Landslide for completely free when you collect more than 1400 Rubble and this is not a bit of a problem, thanks to augment on Landslide combined with Petrify.
The Rumblers stays with us for 69.75s which is enough to clean the entire room of enemies.
Our Petrify (skill) Duration is 31.00s so no need to add more duration to the build, Cone length is 14m so quite OK.
Are we going to use our Tectonics? Yeah... maybe... sometimes... probably not. So... why not replace it? But with what? I'll tell you! ROAR!
Yes, Roar from [Rhino] will buff your Landslide skill damage and in this build Roar will increase your damage for 53%. Very nice!
Pay attention that melee weapons with specific mods affect Landslide damage (including RIVENS).
Atlas - Stat Stick
Suggested Arcanes are Avenger to greatly increase Critical Chance & Energize.
Lastly, some word about his Rubble Armor - Rubble restores Health to [Atlas] if he is injured, if [Atlas] is at Full Health, Rubble instead grants additional Armor that decays over time. The Bonus is capped at 1,500 points, and it decays at a rate of 5 Armor points per second so if you want that bonus to stay with you... just combine Petrify and Landslide skills in battle.