Before entering the room equip yourself with your shotgun.
As the movie ends quickly duck(he can't hurt you when you are like this). Then go to the statue with the obscura painting(not to close or then it will shift it's position) and start firing at brother obscura.
After a few shots he'll not move(you stunned him). Then stand remove your shotgun and grab the painting.
PS: Don't forget to run out to where the water is leaking