Note 21/01/11: I recently helped some friends with this so a few things happened differently than when I did this back in September 2009. The changes have been made below to reflect the new experience.
The video guides are quite useful, but I find there's just some things it doesn't tell you. So here's some advice to be used with them.
For all missions: When you shoot with your assault rifle, tap the trigger as opposed to squeezing it down completely. I find this helps kill enemies quicker.
This can actually be done with four people. Two people will kill themselves before the final keycard is picked up to get the single boat mission. The difficulty will be on hard because it makes no difference to the AI, it just gives you fewer lives. Auto-aim should be on as this makes things a lot easier.
At the start, you have a choice. You can either all go to the helicopter or two can go there and two can go by car.
- If by helicopter: let the pilot go ahead of you and then the three follow behind in the other car. Since the helicopter takes a bit of time to start up, it makes sense for the pilot to get there ahead of the rest and getting everyone organised to get into a car can take a few seconds. Make sure that you know which sides of the helicopter you'll be getting into before you get there.
- If by car: the pilot will have to take another person with them. The driver will not take the default route, but instead will turn right onto the highway. There will be a ramp to contend with as well but if the person knows what they're doing, they should arrive ahead of the helicopter. If you find you aren't making it there before it, then you should all take the helicopter.
Two of the people will be the suiciding team and will be killing themselves before the final keycard is picked up. If you end up all taking the helicopter, two people should jump out just before the helicopter lands on the roof. The game should spawn you somewhere at the bottom of the building and you will now be down a life which will help later on. So either by helicopter only or by helicopter/car, two of you will be at the top and two at the bottom. There will always be a keycard on the roof - this is the one you leave until last to pick up.
The pilot should only really go down as far as level three before heading back up. Really, it's a toss up with the other three players as to what happens but obviously the suiciding team will most likely have cleared the bottom and first floor. Find out which side the pilot will be coming down as it will be important for one of the suiciding team to go up on that side to help the other helicopter member. Things may not always go according to plan, for example, one of the helicopter team may get killed accidentally. You may want to change your strategy during the game if you think this will hinder your chances.
When you get down to just a few enemies left, the suiciding team will want to get to work on killing themselves. This is probably the most trickiest part to time because you don't want to be stuck waiting for them to die. It's just good judgement by the players really.
So the biggest piece of information I have for you is if you have two people left before the 5:00ish mark you should get the boat mission but it really just depends on how quickly and efficiently you kill the enemies, particularly those on the ground floor. It was noticed a few times that if some of the enemies were allowed to run upstairs, it could cause the end mission to change, despite the time you get. Obviously if you're doing this with two people initially, the theory changes. Then you should be allowed to get to around 5:20ish for time and still get the boat mission.
So if all goes to plan, by the time the suiciders have run out of lives, the pilot will be in place to pick up the final keycard left on the roof and the other player will be waiting for the boat. Depending on where the other player is when the keycard is picked up, you could hop on a bike to get to the corner quicker.
After picking up the keycard, there will be enough time for the pilot to fly down, land, get out and help the other player. I find it maximises the chances you have of successfully killing all people in the boat. There are three of them; the driver will be in the middle of the targeting symbols when the boat is coming towards you. So after killing them, calmly get back into the helicopter and fly back to the garage. Don't crash land the helicopter. You're better off landing it and running into the garage. However if you're stuck for time, chance it. When my team originally did it, we got a 6:48 so we had the time there to land.
Difficulty makes no difference and auto-aim should be allowed. Three or four people works best for this one. If you have four, two people will have to get to the helicopters and one will fly to the landing area and just wait. It's best to have two people take on the cops. When the pilot arrives back, land the helicopter away from the plane. There's markings on the ground around the plane so land the helicopter outside the box the plane is in otherwise Petrovic won't get out. I found this to be the main issue with this game mode. It seems to cause Petrovic to become glitchy if you land too close. Other than that it's a straightforward mission.
As a side note for this mission, if you're looking to do this later on for Wanted, note that the host should always stay near the plane and not go for the helicopter. It means the NOOSE guys will drop more money when they die.
--Bomb da Base II--
Again, the same for difficulty being any and auto-aim being allowed. Three but mostly four people works best. This one unfortunately is all about luck. As the video states, the truck spawn to the left is the one that works so this is the one you'll want to get. I would advise splitting the team and taking two cars. It allows you to cover two positions on the truck and gives you a slight bit of control on the direction it goes. It also protects you in case the cops happen to get a good shot and blow up your vehicle. At least two team members will still be alive.
So once your designated team member takes the truck, all of you head for the marker. The truck will be slowest so once the others, in one or two cars get there ahead of it, happy days. You want to be taking off in the helicopter by 2:30 but you can be as late as 3:00. Before you get into the helicopter be sure to know where everyone is going to sit.
Unlike what the video does, the pilot should drop two players on the roof of the bridge where the top bomb is. They will then drop down and be able to attack on both sides when entering the room. The pilot (and if they have another player) will head towards the glitch. Land on the cargo hold where the glitch is to save time running to it. Be careful of surrounding enemies. So one or both of you will try jumping down to the bomb. Whoever makes it will set it, the other will just get off the ship. By now the other players will have set the top bomb and are off the ship already. Now all you have to do is wait for the last player to kill themselves with a grenade and respawn. It'll either be on the ground or on the ship, but just get to the ground as quickly as possible.
YouTube video (not mine) on how to do the glitch shown below.