I played on normal at launch, I'm not sure if they changed something after complaints. Searched for it and these guys apparently ran into him early too:
You need to learn what the same predictable thing is though, of course, it becomes routine once you know it, but the first time you play the game, you end up running into one of these fuckers, they mindfuck your best guy and he proceeds to wipe half your squad.
In my first playthrough I got a decent way into the game, but some bullshit mindfuck guy popped up in a mission and messed it all up and I lost all my best guys in a few minutes. I had no chance to progress with weak guys at that point and ended up restarting the entire game. It felt so damn cheap. At least in Enemy Unknown, I felt like I was strategically beat if I lost, XCOM 2 however, I felt cheated. I still enjoyed 90% of my time with the game but jesus christ the balance of this game is wacky.
"First mission after the tutorial", which would be the second mission.
The odds that your first sectoid encounter wipes half your squad is really quite low. They're usually going to resurrect zombie before they attempt mind control, and even if they do, on easy or normal difficulty their psi strength isn't that high, so it's not a game ending threat (the majority of the time).
Squad wipes are unfortunate, and can happen. There are a lot of ways to replenish your troops, though. You just gotta stick with it and tough it out, or take what you've learned and start over better, stronger, with more knowledge. I realize this gameplay loop can be frustrating to a subset of players, so I don't blame you for that.
What you are missing, however, is that the balance is not wacky. It's not "unfair" or "less strategic" compared to the first game. RNG is present, but it's entirely manageable if you plan your actions accordingly. The trap many players fall into is that they can bet all their YOLO moves on a less than guaranteed chance. If the success or failure of a mission requires you to make sure that Bob the alien is dead, and you use a series of actions that only gives you a 80% chance of success, there's going to be 2 missions out of 10 where you completely fuck it all up because you chose to roll the dice.
A good player knows how to mitigate the RNG and choose a series of actions that will get the percent as close to 100% as possible for the situations that are a matter of life or death.