|Donnel's hardy stat spread makes him a perfect candidate for Spring Xander's Carrot Lance (if you're unfortunate enough to have one.) This build aims to have Donnel become a self sustaining physical wall on the enemy phase that can do some respectable damage as well, just be sure to keep him away from mages. (Plus the carrot just feels so right in his hands)
Weapon: Carrot Lance+ sees far better use in the hands of our favorite farm boy Donnel. His high attack and solid defense allow him to tank hits and hit back with some force, while benefiting from the passive sustain of a refined Carrot Lance. Def refinement is the best choice as it feeds into Bonfire and makes him more resilient.
Assist: Swap is taken to pull Donnel's teammates out of tight spots and tank the hit for them, but really any positioning skill works.
Special: Bonfire is a clear choice due to Donnel's massive defense stat, which will allow him to chunk his foes when it goes off.
A Passive: Close Def 3 is the better choice for Donnel as it doesn't detract from his offensive power, and he'll mostly be seeing action during the enemy phase with this build, but Fortress Def 3 is a solid budget choice if you don't have access to Close Def 3 since his attack is higher than average, or intend to run Close Def 3 as his seal.
B Passive: Guard 3 is the ideal B skill for this build as it keeps Donnel's foes from triggering their specials on him, which let's him last a lot longer against his foes.
C Passive: Whatever fits your team's needs will do for Donnel's C skill. However since he'll likely be in the fray against physical attackers a lot a good choice would be Threaten Def 3, which lets him hit hard in the player phase against any foes that attacked him.
Sacred Seal: There are plenty of solid options for seals now which give the build a lot more flexibility. Quick Riposte 3 is the seal of choice for Donnel, since it lets him ignore his low speed when retaliating and get in two hits. Alternatively you can double down on Close Def 3 and have his defense stat hit insane heights.