User blog comment:B214/Question and Answers 6/@comment-35679297-20191013124044/@comment-33359929-20191013130842

Reflect: Dunno. Reflect has the ability to reflect all attacks, but Penetrate has the ability to byepass all defences. It's a classic case of an unstoppable force meeting an immovable object.

Half Dimension: Depends on how large the Penetrate-imbued attack that Issei sends at him is. Since Half Dimension halves the size of objects and beings around the use (Vali), rather than affecting how strong an attack is.