Forkball said:I think I know this one.
Go across to the blue gel area. Put the beam so it will take you back to the other side, and then hit the gel button. Jump up in the beam, and the blue goo will follow you. When you get to the other side, get out, and stand on the button. It will reverse the direction. Wait until the blue goo is almost at the generator, then portal to a wall above and to the side of the turrets. Then step off the button, and let the gel travel. When the gel is above the turrets, portal somewhere else and let the gel fall on the turrets, killing them. It may take more than one try to get them all.
That one took me a bit to figure out too, good puzzle.