The Giga Mermaid was initially seen chained and under mind control courtesy of the Techno Baron, causing the mermaids to attack Sequin Land. Shantae eventually frees the queen after a long battle.
Pirate Queen's Quest
In the Risky route, the Techno Baron was able to recapture and brainwash Giga Mermaid once more, but Risky frees her again. Risky insists that she's still evil, that she didn't free her on purpose, and that the Giga Mermaid now owes her a favor. Despite these claims, the Giga Mermaid still smiles at her once she's freed, and makes no objection to Risky claiming her next Dynamo component.
Friends to the End
As part of Bolo, Sky, and Rottytops' quest, they end up having to fight Shantae's memory of the Giga Mermaid. Rottytops and Sky ask Bolo to give them a strategy for fighting her, but all he can remember are her looks, frustrating his companions.
Boss Fight & Strategy
Phase 1
Initially, the Giga Mermaid is chained to a pair of massive pillars, struggling in vain to free herself. Using the platforms and chains around the pillars, Shantae must destroy the six locks connected to the chains holding the Giga Mermaid.
To complicate matters, the Giga Mermaid periodically releases electricity-based attacks from the gem on her forehead. Initially, she fires a massive orb that slowly moves towards Shantae; the half-genie heroine will usually have to abandon her attack on the locks to avoid being hit. As the battle drags on, the giant mermaid may fire several smaller orbs that move towards Shantae in short bursts; once again, she should abandon her attack on the locks in favor of leading the projectiles around the arena until they vanish. She can also summon lightning bolts that travel around the arena; Shantae can use the platforms to keep ahead of them.
When the last lock is destroyed, the Giga Mermaid will break free of her chains, destroying the pillars in the process. After a short lull, the second phase of the battle begins.
Phase 2
The Giga Mermaid, still under Techno Baron's controls continues to attack Shantae. She mainly attacks by appearing on either end of the dock and summoning lightning; either a single bolt from her gem or a barrage of bolts that strike the ground. The former is telegraphed by a glow in her gem, while the latter is signaled by markings appearing on the ground; either way, Shantae should make sure she's out of the line of fire. She can also dive underwater and attempt to slam her tail on Shantae; she'll pause just before striking, so use that time to get clear.
In between dodging attacks, Shantae must attack the Giga Mermaid's gem to deplete her health. This is primarily accomplished by using the structures that briefly emerge from the water when the Giga Mermaid appears on the dock; Shantae can climb up the structures and jump at the boss to attack her gem. Spitfire is an effective alternative for dealing more damage in a shorter amount of time, but magic limits could make it difficult to finish the Giga Mermaid in this way.
Hardcore Mode
Giga Mermaid's attacks have been modified to increase the difficulty. In her first phase, she attacks more frequently, and her projectiles move faster. Her lightning attack will now instead strike the top-left / top-right of the stage and will travel in the opposite direction around the stage (counter-clockwise if top-left, clockwise if top-right).
In her second phase her lightning strikes will attack more quickly and the hook/platform stage will be risen from the ground for a shorter amount of time.
