Tips for people grinding 1-3 / 2-1, do not kill or send request to other people for killing the raid boss that you have spawned.

Otherwise you will just keep killing and spawning another one instead of having secret garden.

Leave it there for the full 2 hours or until you stop farming.

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You will have more chances of spawning secret garden too, since raid boss is unavailable for spawning as there is already one

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In my experience, bosses and gardens spawn completely independently of each other. There is no benefit to SG spawn rate from holding on to raid bosses...you'll just spawn nothing if the RNG determined that you were supposed to spawn another boss. Of course you may wish to avoid killing the boss for other reasons, but it won't get you any more SGs.