iOS AFNetwork 3.0: Is there a faster way to send multiple API requests and wait until all of it is finished?

I am currently using the following method to send GET API requests. This method works, but I was wondering if there is a faster way. All I need regarding requirements is to know when all of the Deleted mail has been synced. Any tips or suggestions are appreciated.

- (void)syncDeletedMail:(NSArray *)array atIdx:(NSInteger)idx {
    if (idx < array.count) {
        NSInteger idNumber = array[idx];

        [apiClient deleteMail:idNumber onSuccess:^(id result) {
            [self syncDeletedMail:array atIdx:(idx + 1)];
        } onFailure:^(NSError *error){
            [self syncDeletedMail:array atIdx:(idx + 1)];
        }];
    } else {
       NSLog(@"finished");
    }
}

Edit: I don't care what order it is completed (not sure if it matters in terms of speed), as long as all the API requests come back completed.

You can just send deleteMail requests at once and use dispatch_group to know when all the requests are finished. Below is the implementation,

- (void)syncDeletedMail:(NSArray *)array {

    dispatch_group_t serviceGroup = dispatch_group_create();

    for (NSInteger* idNumber in array)
    {
        dispatch_group_enter(serviceGroup);
        [apiClient deleteMail:idNumber onSuccess:^(id result) {
            dispatch_group_leave(serviceGroup);
        } onFailure:^(NSError *error){
            dispatch_group_leave(serviceGroup);
        }];
    }

    dispatch_group_notify(serviceGroup,dispatch_get_main_queue(),^{
       NSLog(@"All email are deleted!"); 
    });
}

Here you can see all the requests are fired at the same time so it will reduce the time from n folds to 1.