When you open the chest, don't stand in front of it, stand just by one corner and slightly away from it, to give yourself room. Open the chest and clear it, and when you close it, IMMEDIATELY hit "Return" to enter turn-based mode. Turn round and have a good look at the four doors to the room-within-a-room. Pick the one which seems least obstructed, line yourself up, and run for one of those tunnels as if the devil himself were on your heels. You'll take some damage, but you're faster than your pursuers, and you should be able to get away down into one of the caves to rest and heal. Then you just snipe from the tunnels entrance in order to cut the numbers down to something acceptable, at which point you can re-enter the hall and mop up the rest. The ability to "shoot 'n scoot" – retreat while firing arrows at your pursuers – is a great help.