The Portal Opens is the final quest in the City of Portal arc. I did this solo in 5-1/2 hours. Took me four tries (including one try where I did about half of it and then my computer shut down).

The first part, getting to the central room, is possibly the hardest. So many reapers and champions in my first three runs. Final run I got lucky and very few reapers throughout. This whole first part is too close to the entrance so Dimension Door back they'll still be chasing you. I did this without equipment to preserve them for later and most of my deaths come here.

After this the central room, left part and right part are not too hard. Enemies appear in certain sections and when you open doors so I got used to DDoor'ing as soon as or before it happened. Then it's easy to come back and snipe things slowly.

With the final triangle arena the key is to alternate going upper left and upper right, do a few attacks then DDoor out making sure bad guys don't end up near the entrance portal (btw you can't go back through the portal so you have to DDoor to get out of the final arena). No reapers and no orange champions so everything was fairly easy to kill.

Twenty-seven total deaths, I think 16 in the first part then I put on my equipment and did the rest and went through without losing any equipment.