Alien Zombie Megadeath

There are 40 adventure levels, which as the trophy description says, are the blue ones. In order to even complete them, you must first unlock all 40 of them. You'll need to acquire 100 medals to unlock them all.

"Completing" a level consists of killing all zombies. In general, each has about 3 waves of zombies that get progressively harder and more numerous. The medal for completing a level is picture of a rocket against a blue background. You can check whether you've completed a level by just highlighting it on the level select screen – the completion medal will be grayed out if you haven't completed it yet.

The majority of adventure levels are easy to complete and will only take one or two tries. Others are a bit more difficult and will take a bit of skill. I thought the hardest levels by far were 25, 36 and 39. These are tough because there are environment obstacles on these maps that make navigating them a bit tough and there are a fair amount of zombies and bullets on the final wave for each.

See the All the Medals trophy for gameplay tips and strategies, as well as suggestions on what types of medals are easiest to get if you're short of the 100 you need to merely unlock all adventure levels.