Oracle 10g: Optimizing an update query that uses a subquery
Try the below to update unique issuer id and issuer name instead of using Distinct by using Group by together with the index on Issue Id and Issuer Name in Temporary reporting table 

Kindly refer the below link
Fast Query To Get TOP-N Distinct Rows From A Huge Data Table

UPDATE TBL_REPORTING REP
SET(REP.ISSUER_ID, REP.ISSUER_NAME
) = ( SELECT TMP.ISSUER_ID
                   ,TMP.ISSUER_NAME
            FROM   TMP_REPORTING TMP
            WHERE  TMP.CMD_KEY = REP.CMD_KEY
            AND    TMP.REPORTING_KEY = REP.REPORTING_KEY
            GROUP BY TMP.ISSUER_ID
                   ,TMP.ISSUER_NAME
        )
WHERE  REP.CMD_KEY = v_cmd_key;