Trinexx

Trinexx is a giant turtle monster with three necks. The middle neck is the main body, but hitting its head does no damage. Here you first have to beat the necks on either side, and plain attacks won't do it. Find each side neck's weak point and use the item that works on it, and only then do further hits deal damage. Beat both side necks and the shell breaks, and the main body crawls out like a tsuchinoko. Its weak spot is the part glowing red. Get around to its side and give it a blow with the sword.