Spoilers for Book 1/the first four floors

I'm a new GM who has been running AV for a group of 6. They are literally about to fight Volluk, as I've left them on a cliffhanger with his reveal

According to the encounter calculator, a level 7 monster should be a moderate encounter for six level 4 players (they skipped a few encounters so they're just short of level 5), but I also know that fighting things 3 levels above you is really brutal. I was thinking of adding maybe two more low level enemies to balance the action economy, but I don't know if that would be too much.

The party is a Bard, Magus, Barbarian, Oracle, Ranger, and Swashbuckler. Only the Giant Instinct Barbarian and Flurry Ranger have 1 striking runes on their weapons, everyone else just has a 1.

On one hand, I want to make the fight challenging, because it's a boss fight that is important to the Ranger's backstory. On the other hand, I don't want to make an impossible challenge if they get a bit unlucky.

I'd appreciate any advice from more experienced GMs/people who have ran the boss fight before.