If you do an EC vs. EC side battle and you choose the blocker of your hero as one of them and that ec dies do you get a lost soul.
Assuming I understand you right...
You attack with hero, they defend with ec A. You play (sword of the Lord or whatever other side battle card) to make ec B fight ec A. Ec A dies. If no abilities either prevents you from rescuing, adds ecs to the battle, captures or discarsd your hero or some other awful catastrophe occurs, yes you win a ls.