If we take this logic further, and imagine a party gaining a commoner sidekick at level 12, this would add 0 hit dice to the commoner's stat block, since that would be defined as your "starting sidekick level." A level 12 sidekick commoner should not be weaker if acquired at level 12...
When a monster takes its action, it can choose from the options in the Actions section of its stat block or use one of the actions available to all creatures, such as the Dash or Hide action. Melee and Ranged Attacks The most common actions that a monster will take in combat are melee...
How to do large battles Of course you could do it in detail, give everything a stat block and basically do just a large fight like there were 10-30 player characters... but in my experience that offers just a very long and tedious fight. Instead for my games I figured out that the ...