Forrest

Weapon: Dazzling Kumade+ / Dazzling Panic+ / Dazzling Melancholy+

Assist: Physic+ / Restore+

Special: Heavenly Light / Miracle / Flexible

Passive A: Fort. Def/Res 3 / Fortress Res

Passive B: Sabotage Atk / Sabotage Res / Any Chill Skill

Passive C: Res Ploy / Flexible

Sacred Seal: Fortress Res / Chill Res / Chill Def / Def Ploy / Flexible

• This build focuses on inflicting debuffs passively and building Forrest's already great Resistance to an even greater visible number. The latter in particular allows for hitting a greater number of targets with Ploy and Sabotage skills, consequently debuffing and pressuring the enemy team even further. Fortifying Forrest's Resistance to this extent also allows him to tank magic hits reasonably well; however, he struggles to do much back due to a lack of Attack investment and the absence of Wrathful Staff.
• The best ways to build Forrest's visible Resistance lies in Fortress skills and Sacred Seals, including Fort. Def Res or Forrest's default Fortress Res or both (with the latter as a Seal).
• Chill skills are emphasized as a key feature of this build, as they inflict debuffs on the enemy team without needing to do much of anything.
  • As such, Kumade would be the best staff to use if you are able to provide it. Else, Forrest can run pretty much any other staff of your choosing; his default Panic staff is great enough but requires Forrest to initiate in the thick of enemy territory. Melancholy (or its seasonal variant Witchy Wand) are also notable for providing extremely valuable debuffs in Guard.
  • If you are unable to inherit Sabotage skills (as they are available only on 5 star exclusive units at this time of writing), any Chill skill of your choice is typically easier to access (all are available on free Heroic Grail units) and hits opponents true without adjacency requirements / Resistance comparisons.
  • If you wish to place less emphasis on Resistance comparisons (or simply are not using Ploy or Sabotage skills at all for any reason), Chills can be run in both the B and Sacred Seal skill slots.
• The healing Assist and Special don't exactly matter much in this build's scheme of things, and ultimately depend on timing and healing preferences. 

Weapon: Dazzling Pain+ / Dazzling Panic+

Assist: Martyr+ / Restore+

Special: Heavenly Light / Fireflood Balm+

Passive A: Atk/Res Push 4 / Atk/Res Solo / Atk/Spd Push 4 / Atk/Spd Solo / Attack +3 / Equivalent Bond skills

Passive B: Wrathful Staff / Dazzling Staff

Passive C: Hone Cavalry / Savage Blow / Res Ploy / Fortify Cavalry

Sacred Seal: Attack/Res 2 / Chill Res / Chill Spd / Savage Blow / Atk/Res Bond / Atk/Spd Bond / Atk/Spd 2

• A more offensive take on Forrest for those not wanting him to simply sit around and look pretty. As such, Wrathful Staff is required in one form or another, while a Dazzling Staff should accompany it in the B slot or as a refined staff for decent-scale, uninterrupted magic hits against all but Null C-Disrupt users.
• An offensive build, of course, needs offensive additions to it, and much of Forrest's power will lie in his A slot skill.
  •  Skills that increase both Attack and Resistance (Atk/Res Solo, Atk/Res Push, Atk/Res Bond) are generally the best for improving his one-shot damage and already-great magical resilience. Those who would like to improve Forrest's follow-up opportunities and doubles avoidance can instead run Speed-based skills with a Speed Asset. Finally, as usual on budget healer builds, Attack +3 (and its other double stat variations) is simple to access and use.
  • An Attack-boosting Sacred Seal is rather obvious, though Chill Spd and Chill Res are technically more effective stat swings that also provides support passively for allies. Savage Blow can be used to slowly chip down enemies and can be doubly applied with Pain, itself in the C slot, or both.
• Forrest's healing Assist, staff weapon, and passive C slot are pretty much subject to personal preferences.
  • Pain is notable for inflicting splash damage, making Forrests's future matchups much easier, though Panic comes by default and is Forrest's best method of crippling in Aether Raids Defense.
  • If you're running a Push skill (which stops working after a certain HP percentage), Maryr+ is recommended to offset the HP threshold by healing Forrest himself while healing allies for a quite reasonable amount. Though Physic's distant healing has advantages of its own and it already comes by default.
  • On cavalry-heavy teams, Cavalry buffs definitely take priority as Forrest becomes able to provide two meaty bonuses at the same time. Outside of that, Savage Blow is great for inflicting splash damage continuously, while Res Ploy comes by default on Forrest and allows him to affect foes cardinally (unlike Chill Res).