|This build uses the Absorb+ staff, refined to gain built-in Wrathful Staff, in order to let Genny become a unit able to tank in the front lines, and two different setups can be run - one enabling her to counter and tank at either distance (more useful for PvE), and the other focusing on her ability to scoff at even the most powerful mages' attacks (more useful for arena).
An HP bane is ideal for such a build since the other stats are more important. A Defense bane means Genny takes more damage, Resistance means both more damage and less potential to Ploy enemies, Speed means she'll be doubled me everything and Attack hinders her ability to kill enemies. Speed has been chosen as the boon since not only does it get +4 as opposed to +3, it also goes a long way to prevent doubles from a lot of enemies with middling speed.
Absorb+ is the core of this build, allowing Genny to restore 50% of the damage dealt after every attack. This is what allows Genny to be a frontline tank - being able to heal allies after any combat is the icing on the cake. Additionally, as with all other staves, Absorb+ gets two refining choices - one provides Wrathful Staff 3 and the other provides Dazzling Staff 3. Wrathful Staff is the better choice, as Dazzling Staff is pointless on an enemy phase build, and this also frees up the desperately needed B slot.
There's not much to say about the assists. Recover+ and Physic+ are much more reliable than Rehabilitate now (Recover+ heals a guaranteed 31 HP, Physic+ heals 21) and are therefore better choices. I personally prefer Recover+ not only because it heals more but also because Genny won't need to heal at range to stay out of combat, but Physic+ also works well if you prefer it.
Miracle is the best special choice in my opinion, allowing Genny to survive a fatal hit if it's ever necessary. It does come with a cooldown of 5, but she can usually survive long enough to make it ready to activate if necessary. If the rest of the team requires certain buffs those are also good options, but if they don't, the other healer special options are rather underwhelming.
The A and S slot both depend on the purpose Genny should fulfil. Close Counter and Close Defense 3 allow her to tank a surprisingly high amount of physical melees and kill most of them (bar some units with very high Res such as Joshua) in return, while Distant Defense 3 and Atk Ploy 3 help her specialise as a mage killer, all the while staying healthy thanks to Absorb+.
Since Genny's Speed is one of the lowest in the game, Quick Riposte emerges naturally as the single best B skill option. With it she can double any enemy when attacked, with virtually no exceptions since there is no Staffbreaker skill - only armors with Wary Fighter and 25 or more speed can avoid the double.
On the C slot, a Ploy is preferred to make Genny more independent. Atk Ploy reduces the damage she takes from enemies, while Res Ploy allows her to get kills more easily and Spd Ploy makes it harder for some enemies to double her. A Fortify or Hone can also be run to support the rest of the team, but Ploys allow her to safely debuff enemies and get better matchups of her own.
Good teammates for this Genny are those who can provide her with buffs - Performing Arts Azura stands out, being able to dance and give a +3 to all stats with an extra +3 to Resistance from Drive Res, and Oscar is also very good since he comes with Rally Spd/Def and Spur Spd/Def, although any unit with a double rally and spur can support her very well. Both of them also have good matchup coverage against physical units with their default kits, which Genny appreciates in the event of not being able to take down a physical unit. Also make sure to run a unit that can check bow units, especially Brave Lyn, if you don't run Distant Defense and Atk Ploy.