Still Alive Achievement
Complete Course 4 with neither you nor your co-op partner dying
This can be gained in Course 4 – Excursion Funnels Chamber 1-9 and can be gained in either split-screen or over XBL.
This will take some time. You'll most definitely have to go through this course to get all 9 chambers memorized so that you make no mistakes.
Just go into each one, learn how to play it and don't worry about deaths. Set up a plan of who does what in each course so you don't make any portal placement mistakes and you should be good to go. Please remember that this all has to be in one go and cannot be done by doing one chamber at a time then quitting out. So if you die at chamber 9, you'll need to do 1-8 all over again.