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*.