Bill Zebub
“It’s probably Matt Mercer’s fault.”
And that's fine. But does every troll in your game have a random vulnerability, or do the players know what their vulnerability is after figuring it out, and can rely upon that knowledge in the future? Or is it that the trolls show up once, the players figure it out, and then these kinds of trolls are never again seen in your campaigns?
No, they get to know certain kinds of monsters, and those naturally become easier to fight. Part of the fun of the game is encountering similar challenges but finding them a lot easier.
But the big challenges, the ones where each players' contribution really matters, I try to keep surprising and scary.
Unless I feel an urgent need to nova because I'm convinced that the encounter has a high likelihood of TPK, I don't even like to use my big guns until round 2. My DMs like to pull tricks like having enemies that are hidden, burrowed underground, or on the other side of a secret door (or wall, in the case of wraiths). I frequently don't even know what we're facing until round 2.
And then in round two you are counterspelled. Maybe round 3, too.
Besides, holding off for at least a round gives the martials in the party a sense of purpose, and lets the DM feel like he's providing a challenge.
That's why I cluster mooks within the radius of a fireball: to make the wizards feel good about themselves, instead of just getting frustrated that their one action each turn doesn't do anything.