You can kill two birds with one stone here with the Both Of Us Are Back achievement
. The method I used was with two controllers, just complete two acts and once you're on the third act the achievement should unlock once the cutscene has finished.
If you don't have a second controller you'll be able to find people by using 'Looking for Group' and you'll be able to create a session yourself or use TA, whichever you prefer.
TIP: If you're struggling with Act 2 Chapter 3 where you're on the bikes I'd suggest getting someone to help you as that bit is tricky.
If there's anything wrong with the solution please leave a comment down below and I will update it for you!
Hope this helps and happy hunting!