even on that toughest of issues vs even on that toughest issues

[anhnha]

"We found the solution even on that toughest of issues," the U.S. trade official told Reuters in a briefing on a tentative deal struck over the weekend in Tokyo on the proposed Anti-Counterfeiting Trade Agreement.
More:US, EU agree on food names in anti-counterfeit pact-UPDATE 2
What is the meaning of "that toughest of issues" here?
Can I replace the bold phrase by "even on that toughest issues"?

 

[Gillnetter]

"We found the solution even on that toughest of issues," the U.S. trade official told Reuters in a briefing on a tentative deal struck over the weekend in Tokyo on the proposed Anti-Counterfeiting Trade Agreement.
More:US, EU agree on food names in anti-counterfeit pact-UPDATE 2
What is the meaning of "that toughest of issues" here? The Anti-Counterfeting Trade Agreement was the most difficult item (issue).
Can I replace the bold phrase by "even on that toughest issues"? No. The Agreement was only one issue. The plural form would be - Even on those toughest issues.

Gil

 

[anhnha]

Thanks!
Can I rewrite the phrase as follows?
"even on the agreement that is the toughest of issues"

 

[5jj]

Can I rewrite the phrase as follows?
"even on the agreement that is the toughest of issues"

It's not very natural.