Kind of wondering if Shield's Zamazenta is the best pokemon for those going solo, as it gets both Wide Guard and Beheamoth Bash. Since after a certain point it doesn't matter how strong your pokemon are, but more managing barrier piercing with keeping your allies alive (yes I know wide guard isn't perfect but at least you'll avoid complete team wipes). So Zamazenta seems a bit better than Zacian in that regard, and perhaps even Eternatus.
Sadly even the extra damage from the legendaries signature moves isn't enough to just solo raids specifically due to the barrier mechanics. Even at lvl 100, against fully evolved raids, you don't have enough dmg to kill them alone through barriers.
edit for clarification: I'm referring to fully evolved raids with 5-6 barriers. Weaker t5 raids can be bruteforced.
So far I do not consider trading in the Dynamax for breaking 2 shields at once worth the extra raw damage the 3 legendaryes do. Different is if you're duoing, as in that case one person can dynamax while the other provides the raw dmg with one of the leges.
WideGuard sadly isn't really reliable. Even against raids who do have AoE attacks, they also every 2-3 attacks use dinamax moves which are *still* going to oneshot someone else (+ potentially set Sand in case of Max-RockSlide, killing also the sash using AIs).
On that note, against certain ground types that do not have Rock coverage, you can almost hardwall them with FollowMe Togekiss and give the AI more time to do damage that way.
Also, prankster Charm and similar -2 attack or spatk effects (from Grimmsnarl or Sableye or similar mons) can be used on turn 1 to mitigate lot of the damage from certain specific raids.