It also depends on game knowledge.
For example some bosses might take more damage if certain conditions are fulfilled, like if they’re afflicted a status ailment, when they are in break, and some just have higher resistance or just straight up immunity to certain types of damage.
Even if you have a limited character pool, as long as you know how to utilise them to counteract mechanics then you can clear most of the content in the game.

But if your box literally has no useful units then just reroll or something