Immortals Fenyx Rising

This video will show you the location and battles against all 4 Mythical Beasts.

Timestamps:
00:10 Kottos, Mythical Hekatonchires
03:48 Ozomene, Mythical Harpy
09:07 Medusa, Mythical Gorgon
15:21 Polyphemos, Mythical Cyclops

This achievement requires you to kill all of the Mythical Monster Bosses, most of which are located out of the way of the main quest. There are 4 bosses in total that you must kill.

Note: some bosses with the same name may appear in god vaults. They do NOT count towards this achievement. You must kill each of the 4 bosses in their designated areas.

1. Ozomene the Harpy - Valley of Eternal Spring
This is technically the easiest boss of the 4, as Athena will guide you to it eventually through one of her quests. This boss can be found on an island to the Southwest of the Valley of Eternal Spring's mainland. The island has a large cyclone shooting out of it, which is an indicator that you're in the right place.

Ozomene behaves similarly to regular harpies, with a few minor exceptions.
Things to know before you fight:
-Fiery explosive coals spawn around the arena, use them to your advantage!
-When Ozomene falls to the ground, she will spawn some regular harpies to attack you. Be sure to balance your damage and killing the regular harpies she spawns. Don't get overwhelmed!
-Ozomene will fly to a higher area of the island as the fight progresses, you must follow her to continue the fight.

When you defeat Ozomene, you will receive the Wings of the Hideous.

2. Medusa The Gorgon - Grove of Kleos
This boss can be found on an island to the Northwest of the Grove of Kleos mainland. Everything on the island is dead and it is particularly foggy, which are indicators that you're in the right place.

Things to know before you fight:
-This boss can turn you to stone, which slows all of your actions down tremendously, and will do so often. While it doesn't last terribly long, it can be detrimental. Be very careful when she uses this attack.
-Medusa teleports around the arena a LOT. While this can be very annoying, it is relatively easily countered with good timing on your charged moves.
-Medusa occasionally spawns some normal gorgons to distract you. Similar to the last fight, you'll want to dispatch them quickly to avoid getting overwhelmed.

Upon defeating Medusa, you'll receive the Sureshot bow (or a skin for it if you already have it.)

3. Polyphemos the Cyclops - The Forgelands
This boss can be found on yet another island located to the Northeast of the Forgelands mainland. As you draw closer, you can hear the laughs of Polyphemos and there are large purple crystals floating above his arena, which are indicators that you're in the right spot.

Things to know before you fight:
-While this boss can do a lot of damage to you, he is very slow, which makes his attacks relatively easy to dodge.
-Unlike the first two bosses, Polyphemos does NOT spawn additional enemies, which makes this fight much easier as you can purely focus on the boss.
-Avoiding his stomp and sword swing attacks are the most important part of this fight. If you can do that, the rest comes very easily.

When Polyphemos falls, you'll receive the Guardian's Axe (or a skin for it if you already have it.)

4. Kottos the Hekatonchires - War's Den
The final boss you will face is Kottos. He is normally the last boss the player will face as the area he is in is inaccessible until much later in Ares's questline when Ares helps you open the fort. This boss is located in Ajax's fort, which is in the Southeastern part of War's Den. Kottos is located on the rooftop of the tallest building in the fort, marked by glistening spots around the roof. His roars can also be heard occasionally as you get closer, indicating you're in the right spot.

Things to know before you fight:
-This is likely the hardest boss you will fight out of the 4, his ground punch and roll attacks are the most threatening. Be very careful when he uses those attacks.
-Similar to Polyphemos, there are NO additional enemies during this fight, which makes it much more manageable.

When you defeat Kottos, you'll receive the Sprintrunner sword (or a skin for it if you already have it) as well as the "Who's the Boss?" achievement. Congratulations!