As I mentioned in a comment, I wonder if you are confusing Seridei Eish with Aish Kodesh, which contains the sermons delivered by the Piaseczno Rebbe, R. Kalonymus Kalman Shapira HY"D in the Warsaw ghetto.
In particular, in his sermon for Chanukah of 5702, the Rebbe writes:
הן אמת שיסורים כמו אלו שאנו סובלים עתה באות רק אחת לכמה מאות שנים ... אלו האנשים שאומרים שיסורים כמו אלו עוד לא היו לישראל טועים הם, בחורבן ביהמ״ק בביתר וכו׳ היו כמו אלו
It is true that the torments which we now suffer from come only once every few hundred years ... Those who say that Israel never suffered torments like these are mistaken. During the destruction of the Temple and in [the siege of] Betar etc. there were torments like these.
(Translation mine)
However, around a year later the Rebbe added a footnote to this paragraph, which reads as follows:
הג“ה: רק כהצרות שהיו עד שלהי דשנת תש“ב היו כבר, אבל כהצרות משונות, ומיתות משונות, ומיתות רעות ומשונות, שחדשו הרשעים הרוצחים המשונים עלינו בית ישראל, משלהי תש“ב, לפי ידיעתי בדברי חז“ל ובדברי הימים אשר ישראל בכלל, לא הי‘ כמותם, וד‘ ירחם עלינו ויצילנו מידם כהרף עין
[Note:] Only such torment as was endured until the middle of 1942 has ever transpired previously in history. The bizarre tortures and the freakish, brutal murders that have been invented for us by the depraved, perverted murderers, solely for the suffering of Israel, since the middle of 1942, are, according to my knowledge of the words of our sages of blessed memory, and of the chronicles of the Jewish people in general, unprecedented and unparalleled. May God have mercy upon us, and save us from their hands, in the blink of an eye.
(Translation from J. Hershy Worch, Sacred Fire: Torah from the Years of Fury 1939-1942 (Rowman and Littlefield Publishers, 2004), 307n1.)