I wouldn't mind if the rule were changed to something like auto-rout / auto-kill only once a certain number of turns have elapsed without the death (not counting death if rebirthed due to Phoenix Pyre, but deaths leading to resurrection by item do count as none of those are potentially infinite) of a unit that was on the battlefield at the start, or the expenditure of a gem (either of these being basically permanent). That way, as long as permanent damage is being done, the battle continues.
(For a really epic feel, lower the light level gradually and reduce stats accordingly, then raise it again if it Lasts long enough.
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