The reason many people do the hydra inside the black garden then kill themselves is it is a lot faster. It saves a lot of time when you have hundreds of hydras to kill
The same for the shrine of oryx ogre. You can kill it and wile and repeat very quickly
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The Hydras at the Black Garden Gate respawn every 20 seconds after you clear out the four Minotaurs and one Hobgoblin. Very quick compared to some spawn point. And you don't have to kill your self.
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Yes they re spawn fast outside, but you can kill the hydra inside with a shotgun faster then that. It just saves a bit of time