Everything can be done solo.
One is related to FOBs. I am pretty sure that any attack on any FOB online does the trick. As far as I remember, you dont have to win, but if you do need a successfully completed FOB infiltration - just choose an FOB with lower levels of defense to deploy there, some FOBs online are left without any soldiers deployed even, should be no trouble.
I cannot confirm if event FOBs count towards this achievement, they might.

2 achievements to create (first achievement) and dismantle (second) a nuke are a bit tricky to do (I did both a few days ago).
You should plan your timing. Alive nuke is like a target on your back that your base should be invaded and nuke will be stolen by other players once created, community is still active in that area and actively invade bases that develop nuke to secure the nuke-free status in MGSV.

So here is what I did.
Once you start developing a nuke it takes 18 hours in real world. Plan your time to be online at the time it actually finishes development and dismantle it right away.

thanks