Girgashiy in Decisive Battle

I kind of figured it out after looong time... but... I used 30 nao stones and still failed the mission. Maybe it's bad luck. I realised that when stopping the shadow only npc and pets could damage the Girgashiy and the shadow (I'm way too glitchy and laggy to let the shadow be near Girgashiy). The problem was when my npc stopped attacking and just waited for Girgashiy to die of old age :). The other problem was that when i died near Girgashiy just right after my revival he was using that Soul Absorption again and I didn't have a chance to escape or start fighting because the shadow was appearing too close to Girgashiy. Finaly every time I die he regains hp (about 5-6%). Anyway, thank you all for respond and advice.