SQL & PL/SQL :: Query To Return Not Null Column Values With Priority

I have a table with multiple rows for the KEY attribute(its not a primary key) and a Rank for each row.

I want a query which fetches one row per KEY attribute.The row with lesser Rank should be considered. But in-case if the value is null for any column the value for next Rank should be considered.

WITH TMP_TBL AS
(
SELECT * FROM (
SELECT 'A' DUN,'1' RNK,'A21' col1,NULL col2,'A41' col3,NULL col4 FROM dual
UNION ALL
SELECT 'A','2','A122','A23',NULL,NULL FROM dual
UNION ALL
SELECT 'A','3','A32','A33',NULL,'A35' FROM dual
[code].......

DUN is the KEY attribute . RNK is the Rank for each Row. COL1... COL4 are data attributes

The results I am expecting is

DUNCOL1 COL2 COL3 COL4
AA21 A23 A41 A35
BB12 B23 B15
CC12 C13 C33 C14

I want this to be done with SQL only. So I tried various ways but none were successful.Finally I created a Multi Row function row_nvl and it worked.

SELECT DUN,
row_nvl(rownvl_param_type(RNK,col1)),
row_nvl(rownvl_param_type(RNK,col2)),
row_nvl(rownvl_param_type(RNK,col3)),
row_nvl(rownvl_param_type(RNK,col4))
FROM TMP_TBL
GROUP BY DUN

But I don't think my manager will allow me to deploy a Multi Row function .