Finally done with FUBAR. It's really hard, but the hardest part for me is still Chapter 6, the first part when you're alone with only 5 bullets. I made so many stupid mistakes there, and the more it went on, the more frustrated I became, leading to more deaths.
Chapter 13 first part was manageable if you take cover way, way behind. Adams will not die so it's a matter of picking off the first two waves of enemies, and then take out the heavy with the SCAR's explosive ammo. I didn't really have a problem with the yacht part on suicidal, and didn't have a problem on FUBAR either. I ran all the way up the yacht and took out every enemy coming in, while dodging some grenades along the way.
The other part I had major problems with is, strangely, chapter 14 first part. I spent about 2.5 hours there, constantly dying in cheap deaths while trying to save Adams. Taking the bunker turret and then the gatehouse took only a couple of reloads. While taking the bunker, Adams will not die so I just abused him all the way, having him attack everyone on the map. Worked really well!
And for those who're wondering about chapter select, I did my FUBAR run on chapter select, and didn't play the initial part on the chopper at all. The achievement still unlocked. So as long as the FUBAR completion logo is on all the chapters when you select chapter, the achievement should unlock without having to start a new game.