By request, I've added the RR6 bosses to the Favorite Boss Fight question. I've also added Dimension Shredder Yi Sang ahead of time to the Favorite Identity questions. As before, you can update your responses if you want to change your votes.
MeursaultDieci Assoc. South Section 4 Director
Generate 
Erudition on his S1 and S2, or when allies 
Discard a skill. This buffs his Discard effects giving him shield generation and clash power. His S3 consumes all of it to temporarily grant him additional 
Insight, increasing its damage and 
Sinking application. At the end of the skill, Insight automatically resets to 1 (or 3 if the skill kills its target) if it ended up being 4 or higher.
S1 gives some shields if ever it's discarded.
Erudition is reduced when taking damage and completely expires when staggered.
S1 and S2 Discard skills to set Insight to whatever rank the discarded skill was. S1 gets clash Power, and S2 and S3 get bonus damage and Sinking application depending on the value.
Use the discard trick if you want to maintain his Insight level. Generate his defense skill, then use the upper skill (the one that wasn't replaced by the defense skill). This stops him from discarding any skills.
3 +4
60 (+0)Clash: 5-14Damage: 6-23.4Atk Weight:Insight x 5% of Max HP) Shield if this Skill is Discarded
[On Use] Discard 1 Skill of the lowest rank in all of this unit's Skill Slots
[On Use] Gain (Insight - 1) Clash Power (max 2)
[Clash Win] Gain 2 Erudition (once per turn)
Sinking Count
4 +661 (+1)Clash: 4-21Damage: 9.2-46.4Atk Weight:Insight on self (max 15%)
[Combat Start] Apply Erudition equal to Insight on self to 2 random allies and on self (prioritizes allies with Discard that has the least Erudition)
[On Use] Discard 2 of this unit's Skills in ascending order of ranks from all of its Skill Slots
[On Use] Coin Power +1 for every 6 Sinking on target (max 2)Sinking equal to (Insight x 2)
5 +463 (+3)Clash: 6-25Damage: 21.75-89.25Atk Weight:Insight (max 45%)
[On Use] Coin Power +1 for every 6 Sinking on target (max 2)
[Before Attack] Consume all Erudition on self and gain the same value as additional Insight
[Attack End] At 4+ Insight, reset value to 1 (if this Skill defeats an enemy, reset value to 3 instead)Sinking Potency/Count equal to Insight value against the target (how much of that Sinking is Potency or Count is randomly determined)
15 +564 (+4)Clash: 15-21Damage: 0-0Atk Weight:Discard 1 Skill of the highest rank in all of this unit's Skill Slots
[On Use] Gain +6 Aggro to this Skill Slot next turn
x2Erudition to the ally and self (once per Skill, 3 times per turn)
Turn End: next turn, gain Pierce DMG Up and Blunt DMG Up equal to the number of other allies that Discarded Skills (max 3) x5Insight, multiply the above Shield value by 1.5