Well... I managed to kill Arneh by luring his group onto the sands and then own him with wurms. This took a few tries and meant using some distractions from spirits and minions. You basicly have to lure with your entire hero-group + minions. Then run back to the spoor. While your minions are battling/slowly coming to your MM you get your heroes into wurms, flag them far enough back so they don't exit the wurm and hope that Arneh and all the Acolyte's follow you onto the sand and don't stay on the rocky bit.