IMPORTANT: If you Gold medal the last mission you need in Co-op there have been cases where only the host unlocks the achievement despite the others being awarded the Gold medal. If this happens to you do not panic. Replay a few missions, it doesn't matter which and the achievement will pop.
At the end of each mission, you are awarded a medal depending on how well you protected the objective and for boss missions how quick you killed the boss.
*The requirement for the Gold medal on all missions except boss missions is 80% health left on the objective.
*The requirement for the Gold medal on boss missions is time dependent.
Do not expect to Gold medal every mission on your first go. You will need to level up and upgrade your Trench to easily Gold medal them. In reality you should not have any problem with this achievement with the exception of the second last level 'Volcano' and the last level 'Farnsworth'. The latter being a boss battle.
It is recommended that you use machine guns and machine gun emplacements for 'Volcano'. Learn each wave and it should not be a problem.
'Farnsworth' may seem tough to Gold medal but the boss really has three forms...
First form; one weakness on the top of it's head.
Second form; three weaknesses around it's body.
Third form; four weaknesses that dangle from it's body.
If you equip two 'Gungnir' Sniper Rifles and fire them at the same time at a weakness it will instantly explode. The 'Gungnir' is found in the missions 'Oasis', 'Hospital' and 'Port' in the second act 'Africa'. Make sure the legs you are using have the 'Quick Load' ability on them as there is no point otherwise and remember to redeploy after each cutscene. You will only be able to miss about one shot if you want to make the 320s cut off.
Once all missions have been awarded the Gold medal, this achievement will unlock as well as any other medal related achievement.