I think the only reason to go past 20 might be chest farming. It isnt worth it for anything else.
I've gotten all I want from the event, so I wont be farming the forest anymore.
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Even chest farming doesn't yield as many essences as defeating 5 bosses
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I suppose not, but there is that random chance you get a 10 drop on top of the 5. If you were in the infinite loop like OP says, you might get more than a good complete run.