Quantum's Take

Atom saw a significant rework earlier this, taking his kit from utterly unusable to moderately effective against certain Training Simulator levels, and to an extent in PvP modes such as 3v3 Strategic Challenge. The stun on his ultimate, coupled with significant AoE damage prior to his departure from the battlefield, can be the push you need to complete levels you otherwise struggle with. He doesn't need much investment (if any), so he's just a good champion to have in your level sync just in case.

Damage Type

Physical

Sources where available

The Bleed, Motherboxes (Green, Blue, Purple, Yellow), Random Legendary Shard

RavenHrt's Transmute Priorities

HP, Crit RES, ATK

Recommended Legacy Pieces

Duplication Mirror

Not So Huge Hammer

Multiversal Force Upgrades

Small Target - Enhanced

Reduces cooldown of [Small Target].

Exponential Force - Enhanced

Increases The Atom's ATK.

Subatomic Retreat - Enhanced

Increases The Atom's DMG dealt.

Ultimate

Shockwave & Awe

Atom increases in size dealing Physical DMG equal to 500% of ATK, plus an additional 1500 Phyiscal DMG to all enemies and [stunning] them for 2s before exiting the battle

Global Skill

Atomic Cohesion

Increases all Champions' Physical DEF.

Skills

Small Target

Atom shrinks rapidly, increasing Dodge by 18% for 10s

Exponential Force

Atom's ATK increases by 1.5% per basic attack until the end of battle, max of 15%

Subatomic Retreat

Upon defeat, Atom deals Physical DMG equal to 300% of ATK to nearby enemies before exiting and [Stunning] them for 2s