bosses definitely have level limits, though i couldn't say for sure what the best level to be against them is. for some it might be level 100, for others it might be level 7. most likely, early bosses would be easier at level 100 when their level limit is lower and less powerful magic is available, later bosses you'd want to be as low as possible because you can junction better and their limit is higher, and also diminishing returns for leveling (10-50 adds a lot more than 50-100).
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My rivalry with SuperNiceDog turned out a bit one-sided.