Once you've successfully completed an Act on normal difficulty or above, a "Tactical Challenge" mode will be unlocked which can be accessed from the game menu. There's one tactical challenge per completed Act. Once you've completed these challenges... surprise! A sixth one unlocks. These tactical challenges can be compared to the "firefights" in Halo:ODST or "hordes" in Gears of War 2. You'll have to eliminate the enemies coming in different waves. There are 5 waves per tactical challenge. As the number of waves goes up, so does the difficulty... Tactical challenge 6 has proved to be one of the very, very difficult ones.
This will probably be one of the last achievements you will get, as this is the most difficult achievement of them all. It is one of those achievements to be very proud of to have...
I suggest you search in the forums for strategies posted by members who used different methods and/or weapons.
Perhaps these strategies might help you for the most of it, or at least be a good start:
Thread with strategies/videos on challenges 1, 2, 3, 4 and 5
Thread with strategies/videos on challenge 6