User blog comment:Crimson and Darkness/Volume 25 discussion blog/@comment-24053170-20180716184925/@comment-24595619-20180716201349

It doesn't have actual "Infinity" of Ophis even with the King Piece, he wouldn't be able to face it by himself. Issei tried using Ddraigs Ultimate Technique that can burn away a person's soul and Sirzechs used his full power, but Trihexa still regenerated almost like nothing happened.

Besides, there are various Chief Gods, God-Class, Satan-Class beings facing off against the peices of Trihexa all at once. They went as far as to seal themselves for thousands of years to weaken his OP regeneration knowing that would be the only way to weaken him. And Issei would probably be too tried to continue fighting due to how stamina/energy consuming all of his attacks are.