drupal7

Without other modules:

$uid = 777;
$form_state = array();
$form_state['uid'] = $uid;
user_login_submit(array(), $form_state);

To delete nodes:

$type = ['invoice'];
$nids = db_select('node', 'n')
           ->fields('n', array('nid'))
           ->condition('type', $type, 'IN')
           ->execute()->fetchCol();
if (!empty($nids)) {
  dsm($nids);
  node_delete_multiple($nids);
  drupal_set_message(t('Deleted %count nodes.', array('%count' => count($nids))));
}

To detele custom entity type: