Everyone here seems to have had their own Dynamax Adventures partner horror stories, so how about I add one more to the pile?
Round 1, our team had chosen Jigglypuff, Tangela, Quagsire, and I had chosen Swampert. Our two starting options were Miltank and Amoonguss. So of course, our team decides to go Amoonguss, where my Swampert is the only Pokemon who can reasonably damage it thanks to Blizzard and all of our teammates are weak to at least one of its attacks. Turn one the Tangela tried using Stun Spore, which is at least kind of understandable, it's not obvious that powder moves don't work on Grass mons, but our Quagsire also tried using Toxic on it.
Round 2, our team now had Tangela, Amoonguss, Jigglypuff, and my Swampert. Our options were between some Psychic-type (can't remember) and an Alolan Marowak with a scientist at it. So of course, with half of our team weak to it and another teammate unable to do much to it, our team decided to go Marowak. This was presumably because of the scientist, because all three of my teammates wanted to take the random Pokemon it gave you. What happened with that? The Jigglypuff got exchanged for a Whimsicott. So we now had three Grasses up against an Alowak. Fantastic.
The other two rounds were unspectacular but good god. You all probably won't believe me when I say that we only ended up losing two lives that whole adventure...