The boss is a damage check
Most stages close with a boss that has far more health than the trash before it. You do not outlast it by hiding. You out-damage it.
If your weapons scaled, the fight is short. If they stalled, the timer runs out first.
Build for the pool, not the moment
A boss rewards the same thing the run rewarded: coverage and scaling. Weapons that hit hard but rarely leave you exposed when the boss summons adds.
Bring something that clears the small enemies too. The adds clutter the screen and slow your movement if ignored.
Common boss mistakes
Players often swap to a flashy weapon right before the boss. That new weapon is underleveled and underperforms when it matters.
Stick with what you built. The common mistakes page lists the habits that lose runs.