It has 30 health, you'd need someone to shoot it first. But it wouldn't matter, as long as the MEC has high will to avoid MC, it could even survive 2 direct Riftings if need be.
If you upgraded the MECs' close combat capabilities (which anyone who likes facepunching aliens should) then it would take two punches (18 dmg each) to kill an alien with 30 health . You could achieve this with several ways.
1. Rush the alien with two mecs to both punch it once in one turn, with each mec using one action to get to the alien, the other action to punch it.
2. Dash a single mec to get all up in facepunching range for next turn, and then punch it twice in one turn, since punching with a mec on the first action doesn't end the mec's turn thanks to the upgrade. This would require the alien to not run the hell away from a giant angry facepunching cyborg that's standing right next to it. If you don't have the alien trapped in a nice little corner of the room, I guess you could suppress it with everyone else. Dunno if that would stop the alien AI from risking all the reaction shots by running from the mec though.
Making an alien sit right next to a mec for one turn to get maximum punches out of the next turn is tricky. I've never tried it. With melee aliens it just requires a mec to tank some damage. Shooty aliens I've never been able to punch twice. Usually I don't have to, because one punch is enough.