I can't offer much in terms of Squad help (way too many units and buffs to keep up with nowadays), so I'd offer some generic advice:

1. First turn doesn't need any mitigation from my experience, so you can save a Fujin Potion if you want. 50% mitigation is required every turn after that.

2. Hit Lugina with normal attacks to 70~75% HP first to trigger his first Grand Hazard first (remember to guard unit with highest ATK and apply 50% mitigation).

3. Hit both Seria and Lugina to ~50+% HP (with preferably normal attacks or ST ISBBs) next to trigger Libertine but not any of their 50% skills up til Turn 6. By Turn 6, Overdrive Gauge should be filled. On Turn 6, Overdrive the unit with 75% mitigation skill.

4. On Turn 7, whack both units with every single MT BB / SBB you have along with the 75% mitigation UBB + 50% mitigation. You want to trigger ALL their HP threshold attacks here. NOTE: I used Magress UBB here, so all the HP threshold attacks did 1 damage. Not sure if 87.5% mitigation can survive everything.

5. Lugina is not a threat from Turn 8 to Turn 11, but Seria becomes a huge threat with her ATK + hit count buffs. Spam all MT BB/SBB while focusing ST ISBBs on Seria (4 units up, shouldn't be a problem maintaining BB gauges).

6. Karl 2.0 is not a threat (pretty much Trial 001 damage). Normal attack him while keeping up 50% mitigation. Once he summons Selena + something else spam BB/SBB, with any ST ISBB targeting Selena. Selena shouldn't be able to survive the 2-3 turns required from Overdrive to White World (she Overdrives at 50%). Your own Overdrive gauge should be filled by now.

7. Once Karl dies, he transforms and summons Grahdens. Opponent's field should consist of Grahdens, Karl and the Thunder creature. Continue BB / SBB spam, with any ST ISBB targeting Grahdens.

8. Once Grahdens hits his first HP threshold, he talks and sets up mitigation (his own form of Overdrive I guess). OD your 75% mitigator. He then sets up another turn of mitigation. Unleash 75% UBB (again, I used Magress UBB here; not sure if 87.5% mitigation is enough).

9. Now it's simply BB / SBB spam until Grahdens reaches his next threshold. From my experience, OD gauge doesn't have enough time to fill up now (partly because I started spamming BB / SBB blindly instead of tactically normal attacking). Once Grahdens reaches his next HP threshold, use Hero Crystal and repeat the same process.

10. I have heard something about Karl unleashing a powerful AOE when he reaches 10% HP. I used Fujin Potions to finish off Karl from 11% to 0.

https://imgur.com/a/iQ3RI

I hope it helps.