It was an accidental discovery I made one day while playing Shroomite Ranged that became an innovation of necessity I picked up during Legendary mode.
I noticed that as long as I could maintain Shroomite's stealth bonus, Betsy would ignore me... so when I was playing Legendary and trying to farm Betsy's Wings, I used Putrid Scent to get extra breathing room.
Pretty much any ranged attack should work. Summon damage obviously has the highest advantage here since you're boosting your giant stack of sentries as well, but every other archetype has the tools to sling ranged damage at Betsy and outrace her damage against the crystal without being defeated in the process.
You also don't particularly need a Terraprisma, this is perfectly doable on UFOs as well. Ravens and Deadly Spheres should also suffice, but I've never been in a position where I've been actively farming this event without using either Terraprisma or a Chain Gun/Eventide, depending on the level of effort I want to put into aiming.
Finally, 16 kills!
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There's a 17th kill pending, then I'll open all the Treasure Bags at once. Also.. no, this doesn't work for other bosses. The Twins will happily ignore the Crystal and roast you alive.
That's why there's a 17th kill pending. = w=;;
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E: you love to see it. phase 1 of the Plan has been completed