Mora Deck Fragment Help Needed

I have 4 of the 5 deck fragments.

I'm missing the "Slayer's Mora Deck Fragment" and I've been running the infinite archive for multiple hours each day for a few weeks now to no avail.

In the collection screen it says "found occasionally by exploring the multi-dimensional dungeon that is the infinite archive".

Can someone confirm how it's "found"? I've looked everywhere. Killed +100 plus boses, killed loot goblins. I'm sick of the archives because.

I'm really hoping I'm missing something other than the usual crappy RNG.