I assumed you'd automatically explode like the speeder above, but apparently that's not the case.
It turns out there's multiple ways to kill someone in an X-Wing. The first, and most obvious, is to strafe them down with lasers. The second, and equally obvious, is with an old fashioned rocket.
If you're particularly cranky, or masochistic, you can simply kamikaze right through their chassis. Or their rib cage, if you're attacking someone on the ground. And if you're trying to get rid of those pesky ground troops, there's also a fourth way.
You can just flatten them with the underside of the aircraft.
I can't imagine how minuscule the window is on pulling up, but it's gotta be bloody small. I thought any contact with the ground resulted in an automatic explosion -- I didn't know you could rebound off the snow like a bloody bouncy castle.
The poor stormtrooper probably didn't expect it either. "Oh look, an X-Wing's going to crash right into me. Fair enough. They'll die anywa-- wait, what THE HELL" *zoooooooooom*
It's funny that DICE officially lists the achievement as being "trampled". You'd think squished might be more appropriate.