Every day at approximately 3PM GMT, the three difficulty tiered co-op mission change (Regular, Skilled, Hardcore.) You need to first complete the two Initiation mission to unlock the Regular mission, then complete the Regular mission to unlock Skilled mission, and complete Skilled mission to unlock Hardcore mission. Once you have unlocked a mission, it is unlocked permanently, even when missions are changed over.
You need to complete all three missions in a single day within the 24 hour period. It is recommended that you wait until there are convoy missions available for all three missions, as these missions always only include minigun bosses as opposed to the much more difficult mech boss enemy.
The mission themselves typically revolve around leading an A.I. vehicle through the level. The regular missions are not too difficult, but the skilled and hardcore missions should only be attempted in a full team of five players, with everyone assigned to their necessary classes (two medics, three riflemen is a good setup in most situations.) It will also help if you have a few resurrection coins saved up, you will initially receive a free coin every day you log into the game, but more can also be bought with real money from the store.
The three co-op missions carry over the previous difficulty in the daily rotation i.e. today’s regular mission will be tomorrows skilled mission, and todays skilled mission will be tomorrows hardcore mission etc. But the difficulty of the A.I. is retained for each mission tier.