This is one of my favorite game series ever, and considering that I actually did this game 100% on PC, I figure it'd be a disservice if I didn't share my achievement-knowledge of the game with the community. So let's get right into it.
(note: this is all under the pretext you're going for all 1000G)
Difficulty: 6/10. Possibly a 7. The game itself isn't hard, per say, but it is very challenging. If you're not familiar with portals and cannot think critically with them, this game will make you rip your hair out. And the co-op is tedious.
Length: About 25-35 hours. It could be less if you blast through the campaign (which it at least 10, maybe even 15 hours) and have a partner for the entire co-op experience. The length varies substantially from person-to-person but expect to spend at least a day working your way through all of the puzzles.
Playthroughs: This is where it gets tedious. You have to play through the entire campaign AT LEAST once. You may even have to retry certain levels, (see Smash TV achievement). And then you have to play the co-op at least twice. And again, you may have to repeat a few stages.
Online Achievements: There are a whopping TWENTY-ONE online related achievements for a cool 510G.
Missable Achievements: A lot. This will be elaborated on in the following posts.
Glitched Achievements: None.
ACHIEVEMENT / VIDEO GUIDE
(these videos are not mine, and each respective owner is credited)
Wake Up Call - 5 points
Survive the manual override
Campaign achievement, cannot be missed
You Monster - 5 points
Reunite with GLaDOS
Campaign achievement, cannot be missed
Undiscouraged - 10 points
Complete the first Thermal Discouragement Beam test
Campaign achievement
Bridge Over Troubling Water - 10 points
Complete the first Hard Light Bridge test
Campaign achievement
SaBOTour - 10 points
Make a break for it
Campaign achievement
Stalemate Associate - 15 points
Press the button!
Campaign achievement
Tater Tote - 15 points
Carry science forward
Campaign achievement. I did read somewhere that you can advance past this stage without having Potato-GLaDOS on your Portal Gun, but I don't think this will glitch the achievement. When you arrive at the next puzzle, she will be on your gun and you should unlock it.
Vertically Unchallenged - 15 points
Master the Repulsion Gel
Campaign achievement
Stranger Than Friction - 15 points
Master the Propulsion Gel
Campaign achievement
White Out - 15 points
Complete the first Conversion Gel test
Campaign achievement
Tunnel of Funnel - 15 points
Master the Excursion Funnel
Campaign achievement
Dual Pit Experiment - 15 points
Do the same test twice
Campaign achievement
The Part Where He Kills You - 20 points
This is that part
Campaign achievement
Lunacy - 20 points
That just happened
Campaign achievement
Drop Box - 20 points
Place a cube on a button without touching the cube
Credits to RoosterTeeth
Overclocker - 30 points
Complete Test Chamber 10 in 70 seconds
Credits toRoosterTeeth
Good Listener - 5 points
Take GLaDOS' escape advice
Credits to RoosterTeeth
Starts at 0:24
You Made Your Point - 10 points
Refuse to solve the first test in Chapter 8
See video above at Good Listener. Starts at 1:01, it takes close to two minutes for the achievement to pop up.
Pit Boss - 30 points
Show that pit who's boss
See video above at Good Listener. Starts at 2:00.
Preservation of Mass - 20 points
Break the rules in Test Chamber 07
Credits to RoosterTeeth
Pturretdactyl - 5 points
Use an Aerial Faith Plate to launch a turret
Credits to RoosterTeeth
Final Transmission - 20 points
Find the hidden signal in one of the Rat Man's dens
Credits to RoosterTeeth
Scanned Alone - 5 points
Stand in a defective turret detector
Credits to BamblakopzGameGuides
No Hard Feelings - 10 points
Save a turret from redemption
Credits to RoosterTeeth
Schrodinger's Catch - 20 points
Catch a blue-painted box before it touches the ground
Credits to RoosterTeeth
Ship Overboard - 10 points
Discover the missing experiment
Credits to RoosterTeeth
Door Prize - 20 points
Examine all the vitrified test chamber doors
Credits to IGNentertainment
Portrait of a Lady - 10 points
Find a hidden portrait
Credits to RoosterTeeth
Smash TV - 75 points
Break 11 test chamber monitors
This is one of those achievements you want to focus on as soon as you start Wheatley's chambers. Keep an eye open so you don't have revisit too many stages (which can be time consuming and tedious)
From here on out, all achievements that you earn will be through co-op play. Aside from the actual strategy, I STRONGLY RECOMMEND YOU PARTNER UP WITH A PLAYER WHO:
a) Has a mic (you should definitely have a mic too)
b) A decent rig and internet connection, nothing is more terrible than missing a 4-hour cheevo because someone lagged
c) Can actually play the game. Which means neither of you should be completely dependent on some form of a guide to walk you through.
I will do my best to elaborate on all of these as best I can.
High Five - 5 points
Celebrate your cooperative calibration success
Just high five your co-op partner. Not a high four. A high five. Only that'll unlock the cheevo.
Team Building - 10 points
Complete all test chambers in the Team Building co-op course
This achievement is straight forward enough. Remember that you have to complete all the chambers. You will not unlock this achievement if your partner just brings you to the last stage and complete that only. (As a matter a fact, if I'm not mistaken the pairing system wouldn't allow something like that anyway).
Confidence Building - 10 points
Complete all test chambers in the Mass and Velocity co-op course
Again, straight forward. Remember to communicate with your partner at all times. Refer to above note.
Bridge Building - 15 points
Complete all test chambers in the Hard-Light Surfaces co-op course
See Team Building achievement.
Obstacle Building - 15 points
Complete all test chambers in the Excursion Funnels co-op course
See Team Building achievement.
You Saved Science - 100 points
Complete all test chambers in all courses of co-op
It means you've completed every chamber in cooperative play.
Iron Grip - 20 points
Never lose a cube in Chamber 6 of the Mass and Velocity co-op course
This particular achievement requires good communication between you and you partner. Here is a video guide:
Credit to firehurts
Gesticul-8 - 15 points
Perform all 8 gestures of your own volition in co-op
You will unlock most gestures throughout the course of completing the chambers. By the end of the Excursion Funnels co-op course, all 8 gestures should have unlocked.
Can't Touch This - 10 points
Dance in front of a turret blocked by a hard light bridge in co-op
Straight forward, be sure that your partner does not remove the light bridge (in the case they put it up) or you will die.
Credit to CruelTroopOfDoomTV
Empty Gesture - 25 points
Drop your co-op partner in goo while they are gesturing by removing the bridge under them
Okay this is funny.
Credit to ChcBrs85
Party of Three - 25 points
Find the hidden companion cube in co-op test chamber
You do not have to interest with the actual companion cube (it'll be out of reach anyway) so feel free to let go of the 'grab' key. It simply has to enter your POV and the cheevo will unlock.
Credit to ThatGuyTaylor
Narbacular Drop - 25 points
Place a portal under your co-op partner while they are gesturing
As your partner is doing a gesture just place an OPEN portal under them. So make sure it's your second portal and the first one you placed is actually valid.
Professor Portal - 75 points
After completing co-op, complete Calibration Course online with a friend who hasn’t played before
After you've completed co-op, play random matches until you run into someone who hasn't done co-op yet. Or you can organize it with a friend.
Air Show - 20 points
Perform 2 aerial gestures before touching the ground in co-op
A nifty trick for this particular achievement is to have your partner shoot a portal directly below you, and directly above you. As you're making the infinite drop do the two gestures. CHEEVO UNLOCKED! (Portal 1 players will recognize this is similar to the "Terminal Velocity" drop).
Portal Conservation Society - 20 points
Complete Chamber 3 in the Hard-Light Surfaces co-op course using only 5 total portal placements
There are a plethora of ways to accomplish this, so get creative with it.
Credit to SoftlyAdverse
Four Ring Circus - 25 points
Enter 4 different portals without touching the ground in co-op
This is a bit of a tricky one, you'd definitely need to communicate well with your partner. Open two of your own portals (preferably one above and below you), drop into the bottom one, and as you're falling have your partner open a new portal and then replace your bottom one. ALWAYS REPLACE THE BOTTOM PORTAL, or else you will just hit the ground.
Credit to technole
Triple Crown - 15 points
Solve 3 co-op chambers in the Mass and Velocity course in under 60 seconds each
There is no preference. See what chambers you're comfortable with and which ones you know you can do the fastest. The clock doesn't start until you place the first portal.
Still Alive - 15 points
Complete Course 4 with neither you nor your co-op partner dying
This is unbelievably tedious. Excellent and constant communication are a MUST. And if there was a time to stop joking around, this would be it. Your partner may become very agitated with you if you ruin the run. You both have to have 0 deaths to unlock this achievement.
Asking for Trouble - 10 points
Taunt GLaDOS in front of a camera in each of the five co-op courses
Nothing special, at the end of one of the chambers in each course, do any of the gestures.
Rock Portal Scissors - 20 points
Win 3 co-op games of rock-paper-scissors in a row
This cheevo is based solely on luck. You have to keep playing until one of you wins 3 games in a row. You could leave your character to idle while your partner keeps repeating the gesture (RPS is a gesture) and vice-versa.
Friends List With Benefits - 50 points
While playing co-op, hug 3 different people on your friends list
FREE HUGS!
Credit to WhackinPhoenix
And there we have it! Good luck completing Portal 2, and I hope you enjoyed reading my guide