proof of hero

Unfortunately there won’t be an event with proof of hero in the shop until the summer event rerun.

Just start farming here lol
https://grandorder.gamepress.gg/quest/band-corsairs
Apart from Musashi, I'm sure you need plenty of them for your other servants

I think one of Dantes event quests are going to give you about 18 Hero Proofs. Gate 4th.

Aside from that, farm the pirate ship in Okeanos all day until june/july =/

Proof Hell is annoying, since ever1 needs a lot of it, but nowhere near as bad as Crystal Hell, Plume Hell, Heart Hell or Medal Hell. Even joining events, you will never have enough of those. EVER.