execute immediate 'alter session enable parallel dml';

 for x in list_of_partitions_in_T.first .. list_of_partitions_in_T.last
 loop
    dbms_session.set_context(x....) -- значение из list_of_partitions_in_T
    ...
    --формируем merge такого вида:
    l_sql := 'merge /*+parallel(24)*/
                     into (select * 
                              from T
                            where T.date_col = sys_context(....))
                  using (select ...
                              from some_other_table
                            where some_other_table.date_col = sys_context(....))
                  ......
                 '
    execute immediate l_sql;
    commit;
 end loop;

execute immediate 'alter session disable parallel dml';

23  | Id  | Operation                           | Name              | Rows  | Bytes | Cost (%CPU)| Time     | Pstart| Pstop |    TQ  |IN-OUT| PQ Distrib |
28  |   3 |    MERGE                            | T1                |       |       |            |          |       |       |  Q1,06 | PCWP |            |
29  |   4 |     PX RECEIVE                      |                   | 45493 |   562M|  5550  (15)| 00:00:01 |       |       |  Q1,06 | PCWP |            |
30  |   5 |      PX SEND HYBRID (ROWID PKEY)    | :TQ10005          | 45493 |   562M|  5550  (15)| 00:00:01 |       |       |  Q1,05 | P->P | HYBRID (ROW|