Is there any best practice (like deleting bunch of Appian objects at a time by g

AFAIK, I don't think there isn't a way other than deleting the application contents one by one in order to undeploy the application. Even we too come across the same usecase, but we didn't find any effective approach other than deleting one by one.

Also I would like to suggest to maintain the backup in one environment atleast, and carry testing in other environments in order to ensure that everything is working as intended even after deletion of non required Appian objects. In case if there are any failures you could immediately backload the rules from the environment in which you are maintaining the backup. Ignore this if you are sure about the objects that needs to be deleted and the impact they are going to make on the other required Appian objects in the instance.

AFAIK, I don't think there isn't a way other than deleting the application contents one by one in order to undeploy the application. Even we too come across the same usecase, but we didn't find any effective approach other than deleting one by one.

Also I would like to suggest to maintain the backup in one environment atleast, and carry testing in other environments in order to ensure that everything is working as intended even after deletion of non required Appian objects. In case if there are any failures you could immediately backload the rules from the environment in which you are maintaining the backup. Ignore this if you are sure about the objects that needs to be deleted and the impact they are going to make on the other required Appian objects in the instance.