I played Mission a couple of times back when the game first came out but ignored it since then. I had gotten to 24 which was good enough for the Unstoppable title/achievement, and put me at 20,000 or so on the leaderboards. Today I thought I'd kill some time and gave it another shot. I didn't come up with a strategy per se, but here are a couple of spots I found useful:
For normal waves the raised platform in the top right is a pretty good spot. You have some cover, the enemies bunch up mostly along the fat yellow line, and if you are using a delta or riot shield squad they will rappel down right there and stay pretty much along the red line.
For helicopter waves:
Jump onto the air support box, then onto the pile of cement bags next to it and the helicopters won't be able to shoot at you. The enemies that come up along the green line are easy to kill, however the enemies that come up along the yellow line are more difficult. They have some cover and are harder to see, but it's still a pretty decent spot for the early waves.
For juggernaut waves there's no strategy, just run loops and thin out the enemies, and take out the juggernauts at your leisure.
I got up to wave 33 putting me in the top 600 on the leaderboards which really surprised me. Give it a try or two and you'll be able to get a decent rank on this map.
Mission is a
Tier 2 map.
Helicopter waves: 6, 15, 24, 30, 36
Juggernaut waves: 9, 12, 18, 21, 27, 33, 39
