we habe to upgrade production database which is in 220.127.116.11.0 to 18.104.22.168, has around 180GB of clients data, also lot of changes have to do in terms of Table structure and SP for application upgrade also. Those script is taking 3 hours time.
Taking dump and importing into 11g and running script is taking around 16 Hrs, which is huge in terms of downtime.
What will be the best way to upgrade this with minimum downtime with above scenario and RISK free.
Consider using network link with impdp. I've migrated all my databases to Exadata (with version upgrade) using impdp through dblink. This gives you all the flexibility you need to do structure changes as well as excluding unused schemas. You will also automatically resize your db when moving through impdp so if you can afford the downtime you will get allot of benefits by going with this approach.
I'm happy to share my experiences if you end up going with that approach. I'm a bit surprised about the time spent for your migration. My biggest database is 800Gb and that took me 12h with parallel 6. There is several ways to tune this and the documentation is good.