Megaera is one of the main bosses in the game, alongside Tisiphone and Alecto. You actually kill her during Chapter 4 - The Hecatonchires without actually fighting her, but will actually fight her during a flashback in Chapter 20 - The Gauntlet of Apollo. The following is the boss fight in the latter chapter.
Megaera will constantly summon Spartan soldiers during the first phase, but it might help a lot if you kill all but one. Also, make sure you pick up one of the spears for later on, as you will need it. Megaera will only have two attacks in the first phase, which are a ground pound with a slight shockwave, and a combo attack up close. The Plume of Hades move is a great tactic here, as the fist seems to slightly stun and knock her back.
Once you see some green orbs coming from her, it's time for the second stage. The arena widens a bit and any leftover Spartan soldiers will still be here, but there won't be any new ones spawning. She has plenty of new moves here, with the first one being displayed shortly after the second stage begins. She'll hop around on the buildings, then the screen will slow down a bit and she'll pound the ground, sending out a huge shockwave; you can double jump over it.
She still employs the normal ground pound, but also has a new array of melee moves. Do not get greedy with trying to hit her, as the hits cause a good amount of damage. She can do a 4-hit combo, with the first hit being a sideways swipe with her claws, then the ground pound, then another sideways swipe and ending with a forward stab. She can also grab you after bouncing off the walls of the building; escape by pressing the designated button.
Once again, when you see green orbs come out, the second phase is over and the third stage begins. She'll stay on top of the buildings now and summon more Spartan soldiers. She doesn't attack directly from up there, but will send out some of her bugs from time to time. What you need to do is throw the spears at her until she falls down and becomes grappable. Do so to get QTEs to finish this battle.
Right after this, you fight against Tisiphone alone, then Megaera will join in. She retains the same moves as before, except when she teams up with Tisiphone for some attacks.
