Which difficulty you spam on? I did the hardest one, didn't even cost me half hour to complete that hidden reputation.

2nd  to hardest for quicker progress

I dont even know what I'm farming for I wish they gave it to us in a item 

Ouch, you have such bad luck.

I also farm at normal stage and manage to clear everything without wasting too many energy.

If I'm not mistaken the reward is Artifact Rainbow Scale, you can skip it.

But I have such good luck in summon  but still this reputation  shouldnt be rng based I'm now at nearly 2k corals 

Ig this is the curse of someone who pulls limiteds in 20 summons or less I thought not getting  a nat ml 5 was the curse dang

https://epicseven.gamepress.gg/song-sea-event-guide

Cycle through the 3 difficulties and hope you get lucky

1- defeat Scorpetra (same way like you try to find goblin)

2- read it carefully.. it already mentioned the name of the stage (just farm there) *sorry can't remember the stage name since I've already finished it