http://www.kiya1.de/beyond/act2screens/shaman20.png

1. I used sneak with my archer and targeted him as soon as I saw the red cross.
2. As he heals himself, I stunned him with hammer (my lvl was lower than 3)
3. The DK can aid with his 2-hand Death Claw sword.

Important was this => try to lure him away from the water demon. Did this by attacking him and then running back in single mode, where the DK was. As DK was in aggressive mode, he attacked. Took care to heal DK.

Stunning is good, because the Shaman can't heal himself anymore then.