I can't be certain it's bugged but I've never had an event battle be impossible unless it's bugged (eg Jocasta in Knightfall) . The ISB seems to be totally juiced. Despite having the required relics (R10 cassian, r9 for the rest) and even after extensive l remodding, I have been unable to even get through phase 3 of this battle in around a dozen tries (on auto and manual).
Edit: After another dozen tries and more remodding, I finally cleared it. While not definitely bugged (it still might be) and you might claim the battle is working as intended, I don't see how an event tier requiring a massive relic investment requires 50x the time commitment of any other tier level. F2P are collecting their rewards without further investment, remodding, and on full auto. Thank God I 3 star the final tier. Unfortunately I only 2 star the previous on auto.
