Here's my shameless plug i guess.
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Since 2* and 3* wasn't that much difference, RR just walked through it as usual. RR most likely won't be able to 1 shot phase 2 in the future for any newer version though, like 4* and up.
2* HP: 600000/750000/1152000
3* HP: 660000/825000/1270000
Yeah just using my normal teams takes 2 teams to finish it off. Nidhogg still can be overpowered. Some of the nushi or was it prison island? maps seemed harder until you can figure out what they were doing. I especially liked the one that struck first disabling the negate damage.
Oh, does beating the boss give you some special rewards beyond what you're able to farm normally?
Bosses gives 4000 medals the first time you clear them (100 for the first easier one) and nothing after this. But clearing rewards came back each event.
Even if power creep make it easy for advanced players, Nidhogg boss battle is still a hard thing to do. You need a certain amout of rainbows to beat it. Golds are not enough. On my Alt I have 13 rainbows and it was hard to kill the last Nidhogg boss.
It should be easier next time, when you got more power. But if Lavender is your only 6*, you must go on the FG shop and buy Kugaisou for 90 FG. She is a solid Crit 6*.