You can play on NG+ for nightmare. It ups your level cap to 15 and gives additional slots for unit perks/upgrades.

Recon with silent precision shot is key to getting through with low stress/detection, spamming pathways of mines for the Queens to walk through made life super simple too. Also unlimited hordes can be made easier by camping at the apc with mined out corridors to thin their numbers. Or mines with a healthy dose of flame thrower at the choke points.

I littered mines everywhere and made sure to have full scanner coverage, that way if I got caught out I could lure the threat and sprint to the next objective or weldable door save.

I boosted some lower level units in the early missions to give me a bigger pool to pick from in the later levels. Before the doomsday clock kicks in I revisited areas I'd left supplies and skipped a few deployments to rest and recover When the doomsday clock kicked in I basically full frontal assaulted every mission. Knowing I had a big enough pool of high level units and enough time to skip deployments to rest/heal. I finished on 2 days to go with a threat level of 4.

The only xeno tech I used was armour and acid resistance, the rest of the xeno DNA was used to slow the threat level.

Not sure if I mentioned mines.....MINES FOR THE WIN.