Hirotta Koinu ga Cerberus deshita: Jitsu wa Kodai Mahou no Tsuukaite datta Shounen, Honki Dasu to Kowai (?) Aiken to Tanoshiku Kurashimasu

A boy named Sota is betrayed by his party members and pushed off a cliff because he can still only use basic magic, no matter how much time passes. Barely clinging to life, he survives the fall and, at the bottom of the cliff, encounters a lone puppy. But this puppy speaks human language and introduces himself as “Cerberus, the Gatekeeper of Hell.” Half in doubt, Sota goes along with the puppy’s prompting and completes a contract with him, giving his small new partner the name Kel. Despite his cute appearance, Kel is insanely strong. On top of that, Kel tells Sota that the magic Sota has been using is actually lost “ancient magic.” Sota can hardly believe in his own power, so to help him gain confidence, Kel arranges a dungeon‑clearing match between Sota and the party members who betrayed him…

Source: MU