Saving and reloading the level does not work anymore.
I did this by skipping as many of the puzzles needing keystones as possible.
Your main character should be able to fly, such as Wicked Witch. You will still need Batman, Wyldstyle, Batmobile, and Gandalf if you are not using Wicked Witch. Only put other characters on the portal when you need them, or press and hold
to change into them at your position.
Use a character such as Joker to power electric coils. Use a character such as Homer Simpson to Sonar Smash. Use a character such as Jake to enter small hatches. Finally use a character such as Crusty the Clown to put out the fire at the end quickly with Water Spray.
Level 1 - There are no shortcuts, so complete as normal.
Level 2 - Fly to the the electric coils, press the button and then use Joker to charge the right electric coil repeatedly.
Level 3 - Fly to the glass walls, use Homer to smash both of them, then press one of the buttons and quickly switch back to Wicked Witch and press the other button.
Level 4 - Use locate keystone and go left. Use element keystone and press the electricity button, then time it right and press the fire button. Use shift keystone and send a character to cyan, magenta and yellow at the same time.
Level 5 - There are no shortcuts, so complete as normal.
Maintenance Corridor - Fly up and remove the hatch and drop through the pipe.
Level 9 - Use element keystone and use water to put out the fire then earth to grow the plant to get the blue pad. Use locate keystone and go right to produce a dalek that destroys the sentries. Use Wicked Witch/Gandalf to move the red pad into place. Use Jake to go through the small hatch to get the yellow pad. Use shift keystone and go to yellow, press the button and go to magenta and press the button. Use chroma keystone and complete the colour puzzle.
GLaDOS - There are no shortcuts, so complete as normal.
Furnace - Use Crusty the Clown to put out all the fires, starting at the front and working towards the back, then grab the cake!