Li Shenron
Legend
There are many ways to handle this.As for a Rogue hiding behind a column, firing a bow, and then hiding again... I allow it! I see it kind of like gun fighters in a Western Film ducking down behind the horse trough, shooting, then ducking down again. Even though the enemy knows where they are, the enemy is also getting distracted by the barbarian hacking away at them, and isn't using all their awareness to keep track of when the rogue will be popping out again.
First of all, the Rogue can just use her movement to move 5ft out of the column, shoot at no penalty (but also no advantage), then go back behind total cover where she cannot be targeted by almost anything. It is a VERY good situation even without advantage. But it can't be used to represent a western stand-off between 2 enemies only, or it will never end.
Another way is to model the situation as 3/4 cover constantly, buying into the idea that combat is simultaneous so the AC bonus represent the average protection you get while continuously popping out and back without moving much. This is perhaps the best for a stand-off except that it makes it boring (although I must say I actually think western stand-offs are usually extremely boring in any case).
On the other hand I don't see any convincing reason why standing behind a fixed obstacle should ever grant an advantage to your attacks after the first. If you CHANGE the place you're striking from behind, and manage to do so unnoticed, then sure. Otherwise you've revealed your position and for me there is not enough ground to grant a Dexterity check to hide again in the same place.