You can try to create temporary table/subquery:

SELECT DISTINCT t.ret_field
FROM table t
JOIN (
   SELECT '5c8615fa967576019f846b55f11b6e41' AS phash
   UNION ALL 
   SELECT '8719c8caa9740bec10f914fc2434ccfd' AS phash
   UNION ALL
   SELECT '9b657c9f6bf7c5bbd04b5baf94e61dae' AS phash
   -- UNION ALL
) AS sub
   ON t.phash = sub.phash
WHERE t.last_seen BETWEEN '2015-10-01 00:00:00' AND '2015-10-31 23:59:59';

Alternatively do searching in chunks (if query optimizer merge it to one, use auxiliary table to store intermediate results):

SELECT ret_field
FROM table
WHERE phash IN (
        '5c8615fa967576019f846b55f11b6e41',
        '8719c8caa9740bec10f914fc2434ccfd',
        '9b657c9f6bf7c5bbd04b5baf94e61dae')
  AND last_seen BETWEEN '2015-10-01 00:00:00' AND '2015-10-31 23:59:59'
UNION
SELECT ret_field
FROM table
WHERE phash IN ( ) -- more hashes)
  AND last_seen BETWEEN '2015-10-01 00:00:00' AND '2015-10-31 23:59:59'
UNION 
-- ...

If query optimizer merge it to one you can try to use temp table for intermediate results

EDIT:

SELECT DISTINCT t.ret_field
FROM table t
JOIN (SELECT ... AS phash
      FROM ...
) AS sub
   ON t.phash = sub.phash
WHERE t.last_seen BETWEEN '2015-10-01 00:00:00' AND '2015-10-31 23:59:59';