We had our game last night, and the opening combat (we'd stopped last session just before it) went... perfect. I think it's the best combat I've run in my time as GM. It was tense, exciting stuff was happening. Mechanics that hadn't been relevant before were coming up. Instead of the usual Minion Group + 1 Rival, the slaver King-Pin and his inner circle were 6 Rivals + the KingPin. Just adding more targets really up'd the game. I'd also outfitted them with stun pistols, considering their occupation as slavers, so they were hitting the players right in the Strain Threshold.
One character did go down in the first round, but it belonged to a player who wasn't there that session, so he wasn't having to sit out. He was standing out in the open and took a broadside from 3 of them. That's what I noticed about this fight, the players were scrambling behind tables for cover, they were adding little bits of narration trying to get the NPCs setback or themselves boost. They were helping describe their advantages or possible NPC threat ideas, because suddenly every boost, setback, or strain point mattered. One character even had to tactically retreat and do a discipline roll to try and recover some strain so he didn't pass out from the next hit. Yes - a character did something other than attack! That's when I knew the difficulty was spot on. They're realizing shooting isn't the end all be all action.
One player did comment he was shocked by how much harder this combat was. He didn't think it was fair, but seemed to come around when I reminded him of the situation - they'd snuck into this guys inner circle, these aren't street vendors or punk teenagers, but the upper echelon of a thriving slaver organization. I'm a little glad he was shocked, because it means I succeeded in getting the message across - "This isn't Fate Core, not every NPC is a pushover."
All in all, mission accomplished. I think I've broken out of my fear or difficult combats.