I finally played some more over the last two days..can't get past the first Doppelganger.
It's possible to goad him into jumping at you immediately after he recovers from a hit. Experiment a little with backing off when you score a hit and seeing how he reacts. If you find the right timing and distance, you can get him to jump at you where you can score an easy hit every time.
If you still have trouble there's a sidepath in the Marble Gallery that leads back to the entrance hall you can explore. It has a couple of difficult monsters that will result in you gaining a level or two, and I think has at least one lifeup to grab.
The last trick is to go to the bottom of the outer wall where there's a mist grate blocking a room. The room directly above it (guarded by a reasonably powerful knight) has a breakaway wall with an item in it. If you stand in the broken section of wall for about 30 seconds an elevator will appear and take you into the locked room where you can get a strong weapon and armor that should make the fight easy.