need to increase the speed of delete

I have table of 7 crores in 10.2.0.4 database.
 
Application team want to delete the data upto 5 crores and 2 crores need to be table.
 
-->what I said to team is as to give the query to delete 30 lakhs so that i will hit with undo issue.

So they gave queries to delete the data 30 lakhs each time.

this is taking time consuming.Can you please let me know what shold i do to increase the speed.

-->if I alter the table like alter table "table_name" parallel 8 <--I think it may help to increase the speed.

--->i dont think we have option to truncate the data based on condtion as i need to keep another 2 crores out of 7.