I beat stage 455 with the team of T2 Vincemas Mary, A0 Tyltyl, A0 Sienna. You can use any other for the forth/skill tanker. So setup your team as Vincemas Mary for the frontline and keep your Tyltyl and Sienna as your third and forth slot hunters. When Yuri skills, swap to your second hunter. If you're using a tank, quickly heal so that the hunter can tank another of Yuri's skills so you can quickly swap back to Mary to start your finisher/spamming of skill/barrage of attacks. XD
Spam Tyltyl's skill on Mary until you reach the boss. Then spam a barrage of all attacks, starting with Mary to nerf Yuri's elemental resistance. Right after you use Mary's skill, use Sienna's and pray the forth activates. You'll be fine even if it doesn't. But if you don'y have Vincemas Mary, you can still be fine with Shredding Bunny Chick or Windy since activating Sienna's forth negates Yuri's immunity for the entire battle. It also greatly weakens Yuri as well while doing extra damage.
If you decide to use a bomber or support instead of a tank, you'll more likely beat the stage, after Yuri skills once. "Why?" Actually with a bomber or support, you can easily grind and finish before Yuri skills the first time if you set-up jewels and increased gear. Sadly, my Mary has level 0 Gear and no jewels. So I was forced to improvise. No. It doesn't rely on rng. You don't need to proc a 4th if you follow this strategy. :) Just max your skills. Mary and Sienna won't even switch to frontline so just maxing their skills and having them awakened level 1 is good enough. Most likely, aside from negating immunity and stunning, you won't need Sienna. I just spammed Tyltyl's healing and she did all the damage while stacking to Sienna's passive, forcing my Vincemas Mary to crit 90% of the battle.
Still, this is just one of many ways to finish this stage. "Many?" Well I've made many accounts and I always use a different team each time I do this stage. Edward could be used with Themis if you wanted or Themis as a Sienna sub. Anyways, this is how I beat this stage this time around. Good luck everyone.
Sorry for anyone that couldn't load the images. I was rushing to post and didn't feel like writing this since usually my post doesn't save and deletes itself 50% of the time. But I finally wrote it. >:D Have a great day. "If you don't fully understand and I need to be more detailed, I'll edit this post again."
1. Mary, TANK/BOMBER, Tyltyl, Sienna/Themis
2. Spam Tyltyl Heals on Mary until Boss
3. Use Mary Skill
4. Use Sienna/Themis Skill
5. Use Tyltyl Skill
6. Use Mary Skill then quickly switch to second hunter
7 (tank). Tank hit then heal with Tyltyl (wish for proc for quicker finish/3 star)
7 (bomber). Tank hit and die
8. (tank) Tank another of Yuri's skill then switch back to Mary and finish the job with a barrage of skills.
8 (bomber). Finish Yuri with a barrage of skills.
9. (if died). Train your hunters. XD
10. (if died). Spam me hate messages while I read and laugh hysterically. [Optional Step]
11. (if died). Create your own strategy.
12. (if died). Beat the stage
13. (if died). Share your strategy with others in forum or discord.