In the UA cavalry event, for example, Monoma shocked Bakugo with his own usage of Explosion, then defended himself when he borrowed Eijiro Kirishima's Hardening ability.

Source: https://www.cbr.com/mha-sports-festival-arc-class-1-b-quirks-introduced/

Technically speaking, Gigantomachia's initial debut occurs during a flashback focusing on Mina Ashido and Eijiro Kirishima's backstories as a cloaked figure who scares the daylights out of civilians with his mere presence.

Source: https://gamerant.com/my-hero-academia-villains-best-introductions-ranked/