What happen if you're under attack and decided to ignore the message instead? And for example, maybe during the time, you need to do some factory upgrade or some clean-up in other areas? Will you lose the attacked area for good?
Recon: exactly that, recon the area, find the main objectives. Ramble: visit the area again, maybe you miss something hidden, so you can search for it. Cleanup: kill zombies to lower the danger-level Tower-territory: same as factorys, protect the generator to upgrade it, after that you can kill the remaining zombies.
The first mission is kinda annoying as you know squat.
With more areas unlocked, some money starting to flow, you knowing the maps (there aren't many), you'll do it faster and faster despite the number of zombies.
The trick is to unlock all areas before/with the central area and have some lvl 2 factories on 'em to make cash coming in large amount, then it is almost a cakewalk.
Mission 1 or 2 works fine here, just walk around and lure zombies, just make sure you have 15 normal zombies hunt you (no dogs or something else, just normal zombies), then use minigun... or m16... if you can concentreate them on a small spot napalm grenades work too.
Other mission work too, but make sure nothing else than normal zombies are involed, no dogs, bosses, specially no bomb-carrying zombies, kils they make dont count for you.
Yep. Mission1 has just the right number of slow, no-special zombies. Lead them to one point circulating and leading them, then blow them with a napalm grenade.