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."