What is the best way to get the Lament Configurum?

[Naalith]

So early today I finally finished the Weird Tale hunting quests and was under the assumption that I was now at 100% for the game. However, I am missing Bloody Bullets which means I need to activate the Lament Configurum just a few more times. Unfortunately I can't seem to get it. Getting it randomly is hard, I tried doing rainbow runs + quick restart to stage 4 but the rat's 3 weapons stay the same on every reset (whether I die, quick restart, restart the entire game, etc.) and the Rainbow chests aren't dropping it. This is the only item I need to get the finished gun and I'm kinda annoyed. Is there another easy way to get it I'm not thinking of? Can it even spawn in rainbow chests on a rainbow run?

[POIKOIKOI]

no there isnt. its the same with me but with shelleton key. i can never get items that let you steal things, and shelleton key is a good item that is run changing and fun