My wife and I were playing couch co-op in hard mode. Both of us are playing spiritborn.
We decided we would play through the campaign without looking at any build suggestions and just try things out as we go. Our builds are okay, definitely not optimized, so killing some of the bosses near the end have resulted in deaths of one of us at a time here and there.
When we got to the final boss, that was a different story. We died together 5 times, improving on the result of the battle each time. We finally got the final boss to 0% and we got a server disconnected message. This wiped out all of our progress on the final mission.
We were upset to say the least, but we got back to the location again after repeating all of the somewhat time consuming steps. We both died twice to the final boss, and we gave up & shut the game off.
This prevented us from seeing the end of the story and was super frustrating.