Soloing both portals at the same time is by far the quickest and easiest method, IF the people soloing are confident in doing so. All you need is a decent rocket launcher or a full icebreaker clip and the Gatekeepers go down easy. There is hardly any time defending outside and you don't need to worry about the middle. As the OP states, if the soloers are having issues, have one person follow them in and just fire a couple of rockets and immediately run back out before the Gatekepeer goes down. Option 1 = By far the best method. Anyone who says different doesn't know what they are talking about!