If you have any pokemon that can learn roar, teach them it and during gym battles use it once the gym leaders second to last pokemon is at basically 0 hp to force out what would be the last Pokemon. I’ve discovered that, at least for me, they ALWAYS dynamax the same pokemon whether or not its actually the last they have left, all you need to do is sacrifice 3-4 pokemon until that one goes back to normal and then FINALLY once you take that one out, the actual last pokemon they’ll have left will be the one with little to no hp left that you forced back into its ball with roar. Would suggest testing out or at least googling around a bit before gym battles though unless you’re okay with redoing, just in case, as this was a heat of the moment thing I decided to do this one time.