Hard to believe with 〜がたい

Grammar Explanation

怪盗セイント・テール » Volume 1 » Page 74

A student from a lower grade enters the classroom. She stops and stares at Asuka Jr., one of the boys in the class.

「ある特定(とくてい)のところに集中(しゅうちゅう)してない? しかも」
“Isn’t she focused on one particular area? And on top of that…”
“It’s in a frightfully hard to believe area…”