How NOT to Summon a Demon Lord (Manga) Vol. 20

Determined to raise his fighter class levels, Diablo overcomes the difficult trials set by the (supposed) Swordmaster Sasara. He becomes her disciple in no time, and is all set to undergo the final--but most grueling--challenge. Yet before the ultimate trial can even begin, the Demon Overlord Modinaram begins the invasion of Faltra, and our beleaguered party is attacked by...an Evil Ape?