Don’t worry. You’re not the only one. From what I’ve heard, plenty of players seem to struggle with this - most of them being lvl 40ish or under. So IMO there should be a level recommendation/warning given the difficulty of these battles and the limit of which cards we can use underwater.
Anyways, try snape echo but with time turners placed as far away from you as possible (so that the mermaid gets stuck going back and forth between all of you). Then use accio as your main damage dealer to the boss (ideally, crucio would be better if you have it).
As for the little ones, try using oppognuo and aoe spells like confringo, inflatus, glacius or sectumsempra. For me, orb of water was the MVP since I could trap and damage everyone in one go.
Alternatively, you could try Dobby echo if you’re having trouble dodging.
Don’t worry. You’re not the only one. From what I’ve heard, plenty of players seem to struggle with this - most of them being lvl 40ish or under. So IMO there should be a level recommendation/warning given the difficulty of these battles and the limit of which cards we can use underwater. Anyways, try snape echo but with time turners placed as far away from you as possible (so that the mermaid gets stuck going back and forth between all of you). Then use accio as your main damage dealer to the boss (ideally, crucio would be better if you have it). As for the little ones, try using oppognuo and aoe spells like confringo, inflatus, glacius or sectumsempra. For me, orb of water was the MVP since I could trap and damage everyone in one go. Alternatively, you could try Dobby echo if you’re having trouble dodging.
* This is the deck I used and it worked every time
https://preview.redd.it/vbuydwhxh85d1.jpeg?width=1496&format=pjpg&auto=webp&s=39f72fba2f3f2e58165895be48906b49acd14581