Hecatoncheir has a tough shield, so you'd be better off breaking his sword first to decrease the damage he deals, and also his attack range. Staying on his side is also a good idea as his shield is practically invulnerable, but watch out if you get behind him as Hecatoncheir might give you a horse-kick (or sorts) to send you flying off a distance. Also, he likes to jump and try to squish your hero in Rage Mode and slash off a sizable chunk of your HP, so beware.
Hecatoncheir's attacks, taken from his Wiki page:
- Stomp - This is his primary source of attack, the one you'll see most often. He telegraphs this attack by raising his sword straight into the air as if preparing for a march. Hecatoncheir will always stomp twice, and only occasionally follow through with a sword chop. It is recommended to stay on his right (your left) when he stomps, and get a few swipes in on his sword. Once it breaks, avoid the sword all together and target his hind legs.
- Charge - Hecatoncheir will charge at you, sometimes up to three times. Slayers will know he's about to charge when he places his sword across his chest, and it begins to glow white. It's best to just stay out of his way until he finishes. Note: Upon breaking his sword, it will no longer glow white before the charge.
- Centaur Kick - Hecatoncheir kicks you like a horse if you get to comfortable attacking his hind legs. He winds forward before kicking. Just block when he chambers the attack and continue attacking the legs.
- Fire Salvo - Hecatoncheir only uses this attack in rage mode. Fire spurts out of his shoulders on either side of his body at certain intervals.
- Jump - Hecatocheir lowers his body in preperation for a jump. This attack is extremely damaging and potentially fatal. Upon entering rage mode for the first time, Hecatoncheir will begin using this at the end of his stomp combo.
- Rage mode - Like all monsters, upon the first melee duel he goes into rage mode. Fire begins bursting from his shoulders, and all his stats are raised. It is best just to wait out his rage mode, as once he is done, he collapses to the floor for a few moments giving players a chance to punish.
Defeating Hecatoncheir unlocks Yornbea Labyrinth, and you can also do quests in Chapter 8.