How to describe a person who has done well in every task except one, but he has done extremely bad in that 'one' task
Solution 1:
He has an Achilles heel. This implies that his excellence is crippled by a single, titanic flaw.
Apologies for the wiki-link, but the OED didn't capture the full meaning. Link-Wiki
Solution 2:
You could say He has a tragic flaw
A flaw in the character of the protagonist of a tragedy that brings the protagonist to ruin or sorrow.
American Heritage Dicitonary
A more technical (and classical) term is hamartia. Oxford Dictionaries Online
Similarly, the term fatal flaw is also used. Wikipedia
Solution 3:
One might say he has "a blind spot", which in this sense is defined by Merriam-Webster as " an area in which one fails to exercise judgment or discrimination".
Solution 4:
He is just short of perfect?
Or something like... "a blemish on an otherwise perfect record."