Last I checked Atomix was said to be Ben's 2nd most powerful alien after Alien X which scales him above Clockwork's feat of restoring the infinite timelines that were erased by the CTB. Atomix was already weakened which was why Malgaw was able to beat him. Limited Stamina doesn't necessarily mean lower tier, Goku is Multiversal too and his stamina is limited.
Fasttrack (Citrakayah) - Ben 10
7 Low Multiversal:
Feedback (Conductoid) - Ben 10
4 Moon / Small Planet aliens:
Four Arms (Tetramand) - Ben 10
Frankenstrike (Transylian) - Ben 10