The dynamax adventure is very doable, but there's a pretty big learning curve to it. In fact, I've been able to complete runs even before reaching S rank where I can't dynamax my own pokemon.
Some things to note:
1) You can use items (potions, full heals etc.). Use them.
2) Bulky pokemon are almost always better than offensive pokemon. Speed is not very meaningful (in some cases it can even be a downside - what can happen with really high speed is you outspeed them and bring them down to half health, and then they use abilities that increase their speed/reduce your speed effectively taking 2 turns in a row at the time where they take 3 actions per turn, which would've only been 1 action on the first turn and then 3 actions on the 2nd turn if you'd been slower than them consistently), and while offensive stats aren't meaningless, they're just not as important as being able to take hits (especially since you take just as many attacks to remove shields no matter how high your offensive stats are).
3) Try to time when you reduce them to half health when you can. The enemy obviously becomes way more dangerous once they're at half health, and once they're at that point you usually want to just kill them ASAP, and if you reduce them to half with the first pokemon that acts after they do then you can remove 2 shields of theirs with your other 2 pokemon immediately before the enemy can act (or completely remove all 3 shields if you have dynamax unlocked).
4) Pick the long route, not the short one. It gives you way more freedom with which pokemon you fight and in what order, and isn't actually that much longer than the short run in terms of the number of fights (there are a ton of pokemon but you only actually need to fight 3 of them before the boss, any others you fight are optional - you may still want to fight extras if it allows you to get pokemon that have good type matchups or if you want the chests etc. of course).
5) Pokemon that have reflect or light screen are really useful. Obviously a lot goes into which pokemon you pick, if they have bad stats and bad type matchups then of course there can be more important considerations.. but when reasonably possible you should try to get reflect and light screen.
You see this dynamix adventure spawn weirdly against me AI magically spawn in with move that hit 3 of my line and knock 2 of them out on first round tried 400+ times.
There's even issue my game suddenly crash and I lost all my dynamix adventure progress and don't have save options,that's really annoyed living heck out of me.
What I want from dynamix adventure if I unable use my own mon I want to use 1 Mon only so I could switch out if got debuff, when all 3 on field I don't have any strategy I got destroyed by this debuff and it hit 3 times while at it to.
I wish I could customise my Mon stats before start the adventure,reason I want to customise because each time I spawn my Mon stats on wrong spot I have special attack move oddly enough all max stats on attack and to change it I had to go way up to fix the mess up stats.
I don't get single Ultra beast because this spawn issue.
How I reach ultra beast fight not by using protect move or defensive because time way to limited 10 not enough for me ,when I use pure brute strength I manage reach the ultra beast as long non of dynamix spawn with move that hit my entire line on first round or second stage.
I keep failling because I cannot recover HP after battle or PP this pretty much downfall for 400+times when I reach ultra beast.
What I want is using 1 Mon per fight and more time limit.
Funny enough I could easily beat all S rank without healing item you know how? It's because they don't spam debuff and buff each round they don't have shield, I have time to set my next move without worry time limit and no Mon life limit,I can strategize using my own mon because I can relocate my EV point without sacrifice my speed never add EV point on health.
Unlike dynamix pretty much every EV point in Health it pretty worthless for me doing 4 move each turn in second stage that buff I already considered move itself.
Healing and pp recovery already wasting time 2 turn lost just recover alone in battle, dynamix keep debuff me I can't change my Mon to remove the debuff because all 3 on freaking field.
I hate dynamix advanture so much because it's luck base type,I beat all S class and elite six I could beat nuzlock in Yang with hard difficulty it provide yet I cannot beat dynamix advanture what worst it sometimes crash! On wrong time and I lost all progress.
I consider my self beat the game after beat all S class, im not doing ridiculous difficulty luck base dynamic advanture all those mega stone and complete Pokédex can stay there.
It actually rising my blood pressure in real life how annoying it is I rather fight born 1000+ times than going into dynamix adventure.