The idea is to have the guardsman (and sentries if any) as decoy in the direct route towards the defense objective, that way the enemies will always be too occupied to bother with their actual objective.
This then gives you time to respond to each side. I'm only 38 hours in now with a lvl 64 crusader, but I finished all the main story and side investigations yesterday and rushing through the dlc ones atm.
I see someone else already offered you their help, but I'd like to offer my help as well, you can add me https://steamcommunity.com/id/Xeonzs/ and if you see me online just hit me up, I live in the Netherlands though so keep in mind the EU timezones.