Invisibility is what makes boss ending alpha strikes easy.
Invisibility + Haste + Dip/Poison + Action Surge = dead Hag without even needing Giant Strength. The way BG3 gives you a full surprise round after you attack them from invisibility, you get something like 17 attacks before they get to do anything. It doesn't matter how many exactly since it's more than enough to kill bosses. The surprise round needs to be skipped entirely. The initial attack should BE the surprise.
Is there a surprise round in 5e?
The element of surprise has always been a useful feature in D&D, but the Surprise Round, a free round of actions that the party gets before combat that had been a staple since Third Edition, is nowhere to be found in Fifth Edition.
