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.
HeathcliffFull-Stop Office Fixer
Use him with [Hong Lu] Full-Stop Office Rep since he gets free uses of his S2 when Hong Lu uses his S2 and S3. Outside of just doing more damage, the free skill casts lets him build and maintain his {status:Poise} much easier.
Starts battles with 10 
Ammo and 1 
Ammo - Atelier Logic. S1 consumes 3 Ammo. S2 and S3 consume 1 each. His first S3 use is upgraded and uses his only Ammo - Atelier Logic instead. When he runs out of both, he automatically retreats from combat. He 
Reloads when returning (once per encounter), but only reloads Ammo, not Ammo - Atelier Logic.
It is not recommended to use his S1 due to the amount of Ammo spent. Use his defense instead when you don't have a choice. It buffs his next S3 use and triggers [Hong Lu] Full-Stop Office Rep's S1-2.
S2 is a strong single coin skill that inflicts 
Pierce Fragility.
S3 is an extremely strong single coin nuke skill. S3-2 is an even stronger version of his S3. Both versions of S3 consume any 
Poise Potency above 30 to deal even more damage (up to 10 stacks).
His defense skill gives him 1 stack of 
Concentration【Sniper】 increasing his S3's damage and generates bonus Poise. It can stack up to 4. Returning after retreating will also start him off with 4 stacks.
If his passive is active, he gains some 
Haste based on his Poise increasing his S2 and S3's damage (since they get bonus damage based on speed difference). If he did not attack (excluding the bonus S2s from [Hong Lu] Full-Stop Office Rep's mechanics), he grants 3 Poise to an ally and 10 Poise and 1 
Pierce Power Up to himself.
5 +3
61 (+1)Clash: 5-14Damage: 18-39.6Atk Weight:Poise Count
[On Use] Gain 2 Haste next turn (once per turn)
AmmoPoise CountAmmoAmmo
10 +1262 (+2)Clash: 10-22Damage: 24-52.8Atk Weight:Poise and +2 Poise Count
[On Use] Gain 2 Haste next turn (once per turn)AmmoPierce Fragility (2 times per turn)Pierce Fragility next turn (once per turn)
12 +1464 (+4)Clash: 13-27Damage: 61.92-134.16Atk Weight:Ammo - Atelier Logic, activate 'AL-HV Pulverisation Round' instead
If this unit's Speed is faster than the target's, deal +10% damage for every 1 Speed difference (max 50%)
[On Use] Gain 5 Poise
[On Use] Consume all Concentration【Sniper】 on self, gain (value consumed x 2) Poise Potency, and deal +(value consumed x 12.5)% damage
[Attack End] Gain +2 Poise CountAmmoPoise Potency, consume up to 10 of Poise Potency over 30 to deal +(Poise Potency consumed x 10)% damage
15 +1866 (+6)Clash: 17-35Damage: 95.4-209.88Atk Weight:Poise
[On Use] Consume all Concentration【Sniper】 on self, gain (value consumed x 2) Poise Potency, and deal +(value consumed x 25)% damage
[Attack End] Gain +5 Poise CountAmmo - Atelier LogicPoise Potency, consume up to 10 of Poise Potency over 30 to deal +(Poise Potency consumed x 10)% damage
4 +1063 (+3)Clash: 4-14Damage: 0-0Atk Weight:Ammo, this Skill activates as 'Taking Cover' instead
[On Use] If Full-Stop Office Rep. Hong Lu is on the field, request 
Melee Support from that unit (once per turn)
[On Evade] Gain 3 Poise (3 times per turn)
[Turn End] Gain 1 Concentration【Sniper】 next turn (once per turn)
2 +8
63 (+3)Clash: 2-10Damage: 0-0Atk Weight:
Clashable Guard
[On Use] If Full-Stop Office Rep. Hong Lu is on the field, request Melee Support from that unit (once per turn)
[Clash Win] Inflict 1 
Pierce Resist Down (once per turn)
[Turn End] Gain 1 Concentration【Sniper】 next turn (once per turn)
Unbreakable CoinPierce Power Up
When this unit enters the Encounter, gain 1 Ammo - Atelier Logic (once per Encounter)
Turn End: If this unit does not have any remaining Ammo of any kind, activate 
Overwatch Assignment (once per Encounter)
If this unit is Substituted in, or is rejoining battle after Overwatch Assignment, gain max Concentration【Sniper】, 3 Pierce Power Up, and Reload
(Except Ammo - Atelier Logic; once per Encounter)
x2Haste next turn for every 10 Poise on self (max 2)
Combat Start: If this unit is not set to use any Attack Skills (including E.G.O Skills), apply 3 Poise to 1 ally that has the least Poise Potency
Turn End: If this unit did not use any Attack Skills (including E.G.O Skills, excluding 
Target Spotted), gain 10 Poise and 1 Pierce Power Up next turn x5Ammo gains +2 Poise Count when using a Skill that consumes Ammo.
- If the said ally is a Full-Stop Office Identity, or if they have less than 5 Poise Count, gain +4 Poise Count instead (once per turn; does not apply to units without Ammo)