If the damage seems to high to you, lower it! The printed page should never force you to give players a bad time.
Some thoughts as I take a quick look:Spoiler:
Lurkers are tricky. Many of them are designed to work like this:
- first round, hide
- second round, attack for massive damage
- third round, hide. Etc.
Under that scenario, you need higher damage than a monster normally does. This is because the PCs may have ways to reduce the threat.
For the scouts, they should use their quills up front (since it deals ongoing) and then hide. Next round they unleash the brutal attack.
As a DM I would generally use that on PCs that are not yet bloodied, as a great way to soften up PCs and escalate the threat level. The Guards can focus fire on two foes they mark. The scouts spread damage, raising the pressure instead of focusing fire.
If your table usually struggles, try reducing the bonus damage from 4d6 (4-24, avg 14) to 4d4+2 (6-18, avg 12) or even just 4d4 (4-16, avg 10). But, I suspect 4d4 is too low and that it will work fine if you spread out the scouts' attacks.
I like to use the monsters being bloodied as a good way to adjust the encounter. If the PCs are having a tough time, when they bloody a scout I might say "The creature tries to dodge your blow and fails. It struggles to regain its footing, looking tired. You suspect it can no longer strike as hard as it did before." Mechanically, I might take the bonus damage down to 2d6.
Conversely, if the PCs are having too easy a time, when a scout is bloodied I might describe their eyes flaring with hatred and adjust them to be harder (appropriate with this and upcoming encounters). For the scouts, they don't need more damage. Recharging their quill or having them go invisible when bloodied would both be strong enhancements for a very capable table.