Playing the third mission of the game, probably still the tutorial, so... the abject failure that lead to the ownage is quite embarassing.
Anyhow, everybody gets killed except Jack, who is a rookie, and has no grenade. The Alien Relay has been destroyed, bringing in the reinforcements that murdered everyone. Jack has full health (5HP) which is probably the only reason why I continued facing the odds, despite the dude back at command saying to return to HQ.
He is facing a barely injured Sectoid (5/8HP), an ADVENT officer (6HP) and an ADVENT soldier (3HP), and the Sectoid, being a dick, re-animated the corpse of Jack's fallen sqadmate to make a Psi-zombie (5HP, I think) which is... not so hot.
Jack falls back, taking a shot at the Psi-zombie, taking it down to 1HP. Aliens all miss.
Next turn, he runs into a house to make some room to reload, and is panicked by the Sectoid on their turn. He runs out of the house, and hunkers down behind full cover, provide... presumably double cover from doing so. I'm thinkin' "shit, psi zombie is going to shamble up and smash him" as a panicked unit can't take their turn until they lose the panicked status.
Psi-zombie shambles up over two turns, followed by the ADVENT officer, who flanks Jack, providing bonuses to hit and critical chance. ADVENT officer misses twice, and the Psi-zombie also misses whatever melee attack it has.
Jack recovers from his panic, and I decide to return inside the house in order to kill the Psi-zombie. Not the best idea, as I had forgotten the ADVENT soldier was on overwatch. He shoots Jack, reducing his health to 2HP. At this point I feel resigned to losing the mission, so I just want to kill things. Jack kills the Psi-zombie.
The Sectoid moves in the house too, taking position in a large doorway in the house to flank Jack behind full cover, but elects to raise another Psi-Zombie, the bastard. The ADVENT soldier moves up outside, not flanking Jack, and subsequently misses its shot. The ADVENT officer, on the other hand, moves into position on the other side of the large doorway, flanking, and sealing Jack's fate... or so I thought. It missed!
So I decide to run outside the house, opposite my two foes. In full cover at the window, Jack takes a shot that has a 32% chance to hit the Sectoid. Jack succeeds. Jack does 5 damage. Jack has killed the Sectoid, which also elminates the inconsequential Psi-zombie outside!
The ADVENT soldier outside moves up, flanking Jack at the corner of the house, in full cover, but that also misses, while the ADVENT officer moves through the doorway, taking a shot at Jack that fails.
Problem: Jack has no ammunition, needs to reload. Luckily, He is able to climb a drainpipe onto the roof without dashing (which would mean I'd have to wait a turn to reload) and finds half-cover behind a solar panel to reload. The ADVENT soldier follows him up, taking position on the other side of the solar panel... and reloads! The ADVENT officer is nowhere to be seen...
Jack moves along the edge of the roof, around the solar panel to flank the ADVENT soldier, killing it. Disaster! The ADVENT officer runs out of the house, flanking Jack! It aims... but it misses!
Jack jumps down from the roof, behind the full cover of a tree, flanking the ADVENT officer. It has 6HP, and Jack's gun only does a maximum of 5 damage. He has an 84% chance to hit, and a 40% chance of doing critical damage. Jack aims... Jack fires... Jack does 7 damage, killing the final alien to end the mission and save the bloody day.
TL;DR Jack killed 4 units(technically 5, if you include the inconsequential Psi-zombie) on his own - one of which that had more health than his rifle had damage output - under concentrated fire, and survived being flanked so many times it's unbelievable. He is promoted to a sniper, with 7 total kills from his first and only mission.