I don't think there's a built in way of doing this, as ordering is only available for single fields. We just add a field to the query, like "ORDER BY foo ASC". And although we have element summing, that isn't stored in a field in the form's table.
You'd have to maintain your own 'total_score' field, probably with a form PHP submission script, running onAfterProcess, which updates the sum field after any change to the table ...
Code:
$db = JFactory::getDbo();
$query = $db->getQuery(true);
$query->update('yourtable AS y')
->leftJoin('(SELECT user_id, SUM(score) as sum_score FROM yourtable GROUP BY user_id) AS grp ON grp.user_id = y.user_id')
->set('y.total_score = grp.sum_score');
$db->setQuery($query);
$db->execute();
That query should update your whole table. Change 'yourtable', 'total_score', 'user_id' and 'score' to suit your table an field names.
You could add a where() to only update the user_id for this form, but it wouldn't hurt to rebuild all the totals.
Note that if you allow rows to be deleted, you will have to add a second PHP plugin, that does exactly the same thing, running onAfterDeleteRowsForm.
You can then use the new total_score field to order your list by.
-- hugh