Youtuber Antti Kokkonen, who uploads Let's Plays to Youtube under the username Zemalf, is one of the best XCOM players in the world. On January 11, he finished a 50 hour run of XCOM: Enemy Within on Impossible Ironman difficulty without losing a single country. Or Interceptor. Or mission. Or soldier.

It was a perfect run on the game's hardest difficulty (and his first time through Enemy Within). On Ironman, XCOM is limited to a single save file. No do-overs. Beating the game on Impossible Ironman is a rare feat, but beating it without losing a single soldier? That really does sound impossible. But Zemalf did it, and he recorded it all across 58 Let's Play videos.

"I consider myself an okay player, but the run did go really well," he told PC Gamer. With Zemalf's help, we've broken down this achievement in XCOM mastery, dissecting his 58 part series into the key moments that defined the run.

As the XCOM faithful know, Enemy Unknown and Enemy Within operate on a reverse difficulty curve. They're brutally unforgiving in the early missions, when soldiers are inexperienced, panic easily, and die to a single alien blast. "The first five to ten missions are crucial," Zemalf says. "In this run I should've lost, or could've lost, a whole squad in a very early mission where I more or less played really badly. But just out of luck I didn't lose anyone."

If there's a consistent theme through all of Zemalf's run, it's luck. Luck in the missions the game generates, the panic levels he has to contend with, and the soldiers he starts with. And when luck fails him, Zemalf falls back on his slow and thoughtful playing style, methodically considering every option before making a move. He narrates his Let's Plays the same way, speaking in a deep, calm voice.

We've incorporated some of those LP videos below, timestamped to dramatic and crazy and revelatory moments in the run. Watch them as you read for examples of Zemalf's strategy (and some very, very lucky breaks).

Danger zone: surviving the early missions

Chance gets Zemalf out of more than one seemingly impossible situation. Early in the run, he's offered a council mission with a killer reward: five engineers, the equivalent of an entire workshop add-on to XCOM headquarters. He has to go for it. Fifteen minutes into the mission, though, things are looking bad. One of Zemalf's soldiers has already taken a hit and is down to 1 HP. And when he moves that soldier forward to deal with a lone sectoid, he accidentally reveals and activates two more (watch below now) .

At this point, the sectoids get to take two shots at his sniper, who is perilously vulnerable in half cover. They miss, granting Zemalf another turn. He misses his first shot. His second, which needs to kill a sectoid, drops it down to 1HP instead. Then he makes what should be a fatal error: he presses the wrong hotkey, accidentally telling his third soldier—now dangerously exposed—to take a low-percentage shot. It misses.

He stews on it for a minute with only one move left. But that last soldier, the sniper, ends up saving his bacon.