The property you need to set is call "adonet.factory_class". Use any of the following:
<property name="adonet.factory_class">assembly-qualified-name</property>
or
configuration.SetProperty(
Environment.BatchStrategy,
typeof(MySqlClientBatchingBatcherFactory).AssemblyQualifiedName);
or as noted in the readme of the batcher itself (https://github.com/Andorbal/NHibernate.MySQLBatcher):
config.DataBaseIntegration(db =>
db.Batcher<MySqlClientBatchingBatcherFactory>());