Need encounter advice for a large Level 9 party (and how to handle a suspicious min-maxer Gunslinger!)

Hi everyone,

I'm GMing a campaign and I'm currently having a huge problem with one of my players who runs a Level 9 Gunslinger. He has literally become a monster annihilator. Not only that, but suspiciously (and I have no idea how he does it), he is constantly rolling natural 20s... and that's even after I made him change his dice. He's a veteran player who knows all the rules by heart and is constantly looking for legal loopholes to completely break my encounters.

Right now, I need some solid tactics for an upcoming encounter: a fiendish assault on a Gnome farm in Umok. An Elite Erinyes has allied with Graytusk (the NPC from The Fall of Plaguestone, who I am need scaling up to Level 9). The Erinyes has used an "infernal pact" focus spell/ritual to summon a horde of fiends to completely wipe out the farm.

My goal is for this encounter to be not just challenging, but genuinely terrifying for the party. I am completely open to ideas!

Here is my core group (Level 9):

• Forge Dwarf Aasimar Gunslinger: Main ranged DPS.
• Rock Dwarf Cleric of Irori: Main healer.
• Half-Orc Fighter: Main melee DPS.
• Elf Storm Order Druid: Main blaster/spellcaster.

Occasionally, other friends join the table with these Level 9 characters:

• Vanara Farmer Investigator
• Wintertouched Human Sorcerer
• Charhide Goblin Bomber Alchemist
• Catfolk Charlatan Rogue

As you can see, the player group can get very large, which means I sometimes have to drastically scale up the encounters when everyone shows up. The problem is I still haven't figured out how to elevate the difficulty to a point where the characters feel like they are in actual danger, mostly because the Gunslinger just shreds my monsters before they can do anything.

Any advice on tactics, terrain, or enemy synergies to put them on the ropes? Thank you!