miniboss; sub-boss; mid-boss
Video game term for a boss enemy encountered partway through a stage or area, weaker than the main boss.
underboss
Colloquial term for the second-in-command of a criminal organization, often used in yakuza or mafia contexts.
The miniboss in this stage is surprisingly tough.
The underboss of that gang is still young, but they say he's quite sharp.
Compound of 中 (ちゅう, 'middle') and ボス (from English 'boss'). The video game sense is a direct extension of the 'middle boss' meaning, while the criminal underworld sense is a colloquial adaptation.