Ophirion is a Monster type enemy, has 2000 HP, has no minions that support him during the battle, but does heavy damage to a single character when he attacks. The Pterodactyl is strong against Monster types, but you can use other outfits as long as they are not weak against Monster or you use a Creepy Treat card to change your strength type. Additionally, if you are going for Hardcorn Mode, you will need to have one of your team equipped with the Candy Corn outfit.
The achievement's name gives a hint as to the intended way of dealing with this boss.
If you pay attention, you may notice that Ophirion will talk between attacks, saying something prior to performing the attack. He even starts out with a free shot early on. This is the developer's way of showing you how this battle will go. The boss's comments will be directed at a specific character, such as shaving the Wolfman, or extinction in reference to the Pterodactyl. He will then go and attack that party member. At this point, you need to use a Counter Attack, which is learned by training with Corvus when it is made available.
Doing so will damage the boss's armor, breaking a piece of it (kneepads, shoulderpads, arm braces) as well as damaging the boss. In this way, even Candy Corn can do damage, if it is the one attacked. Counter-attacking correctly, plus one or more Creepy Treats to help you out should make this boss fight much easier.