I checked the battle - indeed the Niefel had a Ring of Tamed Lightning, that I didn't notice at first, so that he was not targetted by Bolts makes sense...
Apologies to have implied the AI was so stupid has to not cast Bolts at him out of sheer stupidity
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Well, it's not clear why the ghosts wolves/ph warriors didn't go at him - maybe even they did try to but he was flying, making it difficult to reach. Overall my army was mainly powerless against him, the only sensible action of the seraphs should have been to leave
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No real "bug" indeed...