If you define your order by clause with the DESC option and then use FETECH FIRST 1 ROW ONLY in the select statement, I assume that would get the row you wanted without reading all the rows (if I understand your question correctly).
However, DB2 "may" need to materialize the answer set in a temp table first and then sort the results in descending sequence. If this were necessary it would take DB2 some time and resources to do this. But if you use the ALLOW REVERSE SCANS on the CREATE INDEX you should be able to alleviate that problem.
SELECT MAX(INDEX_FIELD1 CONCAT INDEX_FIELD2)
WHERE INDEX_FIELD1 = 'VAL1' AND INDEX_FIELD2='VAL2'
But I really don't know how that would perform. It's possible that DB2 would go straight to the last entry in the composite index and retrieve the last row. I know it would work well on a single column index. You could do a visual explain and some testing to find out.