but with intuition(3) is toooooooo overpowered (you now why)
You know what would be even more overpowered with intuition(3)?
A simple non-unique rocket launcher.
Sounds a bit overpowered
Compared with a normal shotgun? Certainly.
Compared with other uniques (like, say, Missile Launcher)? I'm not so sure.
First and foremost, it is situational. One can't always rely on having walls and doors to shield him or her from harm; on open ground, Phase Shotgun would be vastly inferior even to a mundane double shottie, let alone other uniques.
Furthermore, as Visalos has pointed out, range will still be a factor. While this weapon may kill the foos who have lined up against the wall, it's useless against those who stand in the middle of the room.
Finally, there's the omnipresent chance of having a barrel (or five) on the other side of the wall. Those who fire Phase Shotguns through the walls blindly are playing Martian Roulettte with live ammunition.