Amai Mask is speculated to emerge as the strongest villain in "One Punch Man" Season 2. Garou will also be Saitama's arch nemesis who will give him some serious trouble. Also, the plot spoilers for the premiere episode suggests that Lord Boros will be successful in counteracting Saitama's punches. 

The weakest link Amai Mask is speculated to emerge as the strongest villain who could defeat Saitama in the upcoming "One Punch Man" Season 2. Because of that, the main protagonist is forced to unleash his most powerful punch to date called the "serious punch".

Amai Mask happens to be a Class A Rank 1 hero who has the highest rank in the Hero Association. He has immense speed abilities which makes him unstoppable among the S Class heroes, Cross Map reported.

Aside from Amai Mask, Saitama will be facing an evil character who will bring him in some serious trouble. According to Parent Herald, Garou aka The Human Monster will cause a lot of troubles to the main protagonist in the sophomore Season. Garou happens to be the former student of Bang or Silver Fang who was expelled from his dojo from being involved in a rampage.

He is described as a young man sporting a long and spiky white hair, yellow eyes, sharp features, and a muscular body. He is viewed as a Mysterious Being in the Hero Association who aims to defeat the top ranking heroes of S amd A-class.

As for the premiere episode spoilers of "One Punch Man" Season 2, Breathe Cast notes, "Lord Boros will be successful in counteracting Saitama's powerful punches. Still, Saitama will remain victorious as he releases his serious punch".

As of now, the production for the anime adaptation is currently underway. The official scheduled launch for "Once Punch Man" Season 2 has not been released just yet. However, the anime series creator speculated that the upcoming installment will highly likely be released around the latter part of 2016 or in early 2017.