Here's my suggestion, based on the actual circumstances that earned me the Specialist achievement.
Play defense on pl_badwater. Make sure your team has a few competent damage sponges (Heavies, Soldiers, and Demomen) who know how to retreat (you may need to explain that bullets are hazardous to your health).
You want to stay alive. You can facilitate this by always sticking to a buddy. The best buddies to stick to are the ones that hang around sharp corners near narrow corridors:
In these locations, you and your buddy can stay behind cover in the yellow area, and once they are fully healed (to 150%, conditions permitting), they can venture into the red zone for ideal damage exchange along the highlighted lines of sight (where Heavies, Soldiers, and Demomen have a captive audience, so to speak, of the cart's escorts).
Note that the general advice here (be on a good team, play against a bad team, heal everybody) still applies, and that you'll still need classes like Engineers (who can help by putting dispensers near you) to cover your back.
The zombies with grenades are the best "weapon" since grenade glow in dark and you can take them with gravity gun packing some chaos...for other tricks check video.
Also Try to remember where the flares are,once it gets dark if you manage to find them you can use them to ignite zombies and get yourself some more light on the level.
Best Answer
There are 511.
I made the following spreadsheet, which might be of interest to somebody who wants to track their progress: http://www.thunderpeel2001.com/GameFAQs/MGS2-Missions.xlsx
Top tip: These are some amazing tutorial videos by user "Dat Boi Treezy" for getting past the tougher levels, too: http://bit.ly/VirtuallyImpossible
Here's the complete list I put together:
RAIDEN
VR MISSIONS
Sneaking - Sneaking x 10
Sneaking - Eliminate All x 10
Weapon Mode x 40
First Person View Mode x 5
Variety Mode x 7
ALTERNATIVE MISSIONS
Bomb Disposal Mode x 5
Elimination Mode x 10
Hold-up Mode x 10
Photograph Mode x 6
Raiden Total = 103
RAIDEN (Ninja) (available when Raiden is 50% or higher)
VR MISSIONS
Sneaking - Sneaking x 10
Sneaking - Eliminate All x 10
Weapon Mode x 5
Variety Mode x 1
ALTERNATIVE MISSIONS
Bomb Disposal Mode x 5
Elimination Mode x 10
Raiden (Ninja) Total = 41
X RAIDEN (available when Raiden and Raiden (Ninja) are both 100%)
VR MISSIONS
Streaking Mode x 1
X Raiden Total = 1
SNAKE
VR MISSIONS
Sneaking - Sneaking x 10
Sneaking - Eliminate All x 10
Weapon Mode x 40
First Person View Mode x 5
Variety Mode x 7
ALTERNATIVE MISSIONS
Bomb Disposal Mode x 5
Elimination Mode x 10
Hold-up Mode x 10
Photograph Mode x 7
Snake Total = 104
PLISKIN (available when Snake is 50% or higher)
VR MISSIONS
Sneaking - Sneaking x 10
Sneaking - Eliminate All x 10
Weapon Mode x 40
Variety Mode x 2
ALTERNATIVE MISSIONS
Bomb Disposal Mode x 5
Elimination Mode x 10
Hold-up Mode x 10
Pliskin Total = 87
SNAKE (Tuxedo) (available when Snake and Pliskin are both 100%)
VR MISSIONS
Sneaking - Sneaking x 10
Sneaking - Eliminate All x 10
Weapon Mode x 40
Variety Mode x 2
ALTERNATIVE MISSIONS
Bomb Disposal Mode x 5
Elimination Mode x 10
Hold-up Mode x 10
Snake (Tuxedo) Total = 85
SNAKE (MGS1) (available when Snake (Tuxedo) is 100%)
VR MISSIONS
Sneaking - Sneaking x 10
Sneaking - Eliminate All x 10
Weapon Mode x 40
Variety Mode x 3
ALTERNATIVE MISSIONS
Bomb Disposal Mode x 5
Elimination Mode x 10
Hold-up Mode x 10
Snake (MGS1) Total = 88
Total VR Missions = 358
Total Alternative Missions = 153
Total Missions = 511