I just alternate between Eternatus, Sword-dog, and any level 100 legendary I have since those can all dynamax (make sure to give them ten of the candy or they wont do as much damage). The first two can't dynamax but they have a move that does double damage on pokemon that do, combine that with sword dog's ability giving him a +1 right away and you'll do a lot of damage. With Eternatus, give it a choice specs and spam that anti-dynamax laser beam. With normal legendaries do the same but with dynamax and a super effective STAB move. I get a lot of use out of Mewtwo and crown goat riding the ghost horse. You'll still get screwed over sometimes by NPCs sometimes though. Sometimes it's better to just run and restart with a fresh set of NPCs. Just keep hitting. Though if you DO have a friend to bring along, have one of you use a dynamaxed legendary and the other use Eternatus or a Sword/Shield dog. It's a good combo.
Two people is usually enough to account for NPC stupidity in dynamax adventures as well. As one guy though, it's all up to luck. I once got through one with three mostly stupid NPCs because nothing we fought could handle the imposter ditto they gave me at the start. It handled the legendary on it's own.