4. Co-op story walkthrough
For this part of the game I recommend a good friend which has at least a slight knowledge of the game since otherwise it will be a pain to do. Both of the players should make sure they know exactly which portal to place where and teamwork is the key for success. Also make sure to be on each other's friends list. Having a second profile for the "Professor Portal" achievement is also HIGHLY recommended, but actually isn't needed.
First off the game starts in a little video sequence where you see both players being attacked (yeah, you don't play as humans). After some short calibration where you basically have to use the ping tool, gestures and after your first little co-op testing chambers, you will get your first achievement. Now you will be able to start the co-op mode for real!
Celebrate your cooperative calibration success
The hub is basically your starting and meeting point, something like a lobby, where you can enter the various test chambers. Don't be mistaken, some test chambers have to be unlocked through gameplay, so don't be confused if you can only see/reach about 3 chambers at the beginning.
Before we start any chambers, we will hammer out a few achievements but you can get these anywhere. Browse through your gestures menu and stop on the rock-paper-scissors gesture. This will unlock after some missions, but if your partner already has it, he can initiate it as well. Simply win 3 times in a row and the achievement is yours. This is purely based on luck and totally random in my opinion, but the game will grant it to you just keep trying.
Win 3 co-op games of rock-paper-scissors in a row
Now open a portal underneath your partner, let your partner make a gesture and place the 2nd portal somewhere else to break the gesture and net you another achievement:
Place a portal under your co-op partner while they are gesturing
Now for using all the portals without touching the ground. One player opens a portal on the ground and puts the 2nd one close to it. Jump into it from some height to gain momentum. There are 2 ways to continue: either be fast and replace the portals with your own as the non-jumping player passes through them, or place 2 portals close to the previous ones so that the jumping player can reach all 4 without touching the floor in between. If done right, the achievement for doing that should unlock:
Enter 4 different portals without touching the ground in co-op
Keep the portals and jump in either set of them. Now press up on the D-pad to initiate a gesture in the air. Do this one more time and another achievement will unlock:
Perform 2 aerial gestures before touching the ground in co-op
After some short orientation head for the first test chambers which should be called "Team Building". On the first stage, right before the end, make sure to do your high-five gesture (ANY gesture will suffice!) in the camera to go toward "Asking for Trouble". Usually at the end of the first chamber in each stage there is a camera to gesture at. Keep that in mind.
And after those 6 easy opening stages you get:
Complete all test chambers in the Team Building co-op course
I found a few videos to help those that struggle.
Now we move on to Course 2: Mass and Velocity. On chambers 1-3, use the following video to complete all three chambers in under 60 seconds:
Solve 3 co-op chambers in the Mass and Velocity course in under 60 seconds each
Again, don't forget to gesture for the camera; Asking for Trouble is nearing up on us! On to chamber 6, follow the video:
Kudos to Pullen101 and SPREEproductions, respectively.
Never lose a cube in Chamber 6 of the Mass and Velocity co-op course
Then after you complete all the other stages you get:
Complete all test chambers in the Mass and Velocity co-op course
Here comes Course 3: Hard-Light Surfaces.
Upon getting to the very first room with a light bridge and water, attach it anywhere you see fit above the water, let your partner walk onto it and have him perform any gesture. Now put the bridge somewhere else and he will fall to his death for an achievement:
Drop your co-op partner in goo while they are gesturing by removing the bridge under them
I also found a video to specify it a little bit more and I don't take credit for it.
Now get to chamber 3 and beat it with no more than 5 portal placements. This is easily done if you know how and in case you should get stuck, I have found a video for that.
Complete Chamber 3 in the Hard-Light Surfaces co-op course using only 5 total portal placements
Now simply play through the rest of the chambers for the achievement. Don't forget to gesture in front of a camera at least once per chapter! I also found some videos which I don't take credit for.
Complete all test chambers in the Hard-Light Surfaces co-op course
Now we REPLAY chamber 6 and place a barrier in front of a turret. Use the dance gesture and taunt the turret and it will result in a neat achievement for you.
Dance in front of a turret blocked by a hard light bridge in co-op
Course 4: Excursion Funnels is our second to last course. Almost done! Use this video courtesy of Community Achievement Hunters Nand and Soothsayer57 for Still Alive, which requires you to do the ENTIRE course without dying. This isn't cumulative, which means if either of you die once, you have to start over from the very beginning, so be careful.
Complete Course 4 with neither you nor your co-op partner dying
The next achievement requires you to be alive the whole course and should unlock at the same time as Still Alive. Don't forget the gesture at the camera!
Complete all test chambers in the Excursion Funnels co-op course
Now we REPLAY chamber 9 and search for the companion cube. Follow the video provided down bellow and you shouldn't have any trouble finding it. I again don't take any credit for it.
Find the hidden companion cube in co-op test chamber
Alright. We've made it. Course 5: Mobility Gels. As soon as you reach the camera of the first stage, make a gesture to unlock a hard earned achievement:
Taunt GLaDOS in front of a camera in each of the five co-op courses
Now stand still and perform every single gesture you are able to perform. This will net you another achievement since you should've unlocked each available gesture (you will also earn 1 hug towards another achievement!)
Perform all 8 gestures of your own volition in co-op
Breeze through the rest of the chambers and sack the achievement for beating the game once and for all!!
I found another set of videos for those that need them, again, i take no credit!
Complete all test chambers in all courses of co-op
Now Professor Portal requires you to play Calibration Course online with a friend who hasn't played before. That means they have to have never played co-op AND have to be on your friends list. They cannot have completed Calibration Course or have been in any chambers. It must be done online, split-screen doesn't count. Also, you must have beaten the coop and been awarded You Saved Science. If all of those requirements have been met congrats:
After completing co-op, complete Calibration Course online with a friend who hasn’t played before
Hug the player afterwards and you should now 2/3 hugs. Get online and search for any player, hug him once and watch your final coop achievement unlock before your very eyes. What a tremendous sight!
While playing co-op, hug 3 different people on your friends list
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