I say Grima!Robin. Grima!Robin has his breath attack, Expiration, an attack he can throw endlessly around which levels cities and towns at full force, alternating between a deadly blast of energy and ruthless spikes which impale the opponent. He's able to summon Risen, ranging anywhere from piss-poor barbarians to snipers, sages, sorcerers- point is, Grima has a lot of area covered thanks to Risen versatility since they spawn armed, even if we go off of base stats for classes.
Furthurmore, Grima!Robin is basically Robin, possessing all of Robin's tactical prowess, godlike stats, and more importantly, powers (namely Ignis, which allows him to enhance his physical strikes/ magic attacks with the opposite stat). Dragonskin allows him/her to halve damage from ANY attack, reduce the effects of skills, and straight up ignore the effects of instant kill skills and non-battle skills that would hinder him/her. Also, the Pasive and Aegis skills wreck even more havoc, both reducing damage by from melee/range by 50% and stacks on top of Dragonskin, as Dragonskin is a passive ability. Hawkeye ensures that Grima never misses, and Rightful God allows him/her to activate Ignis much more easily, increasing activation rate by a flat 30%. Finally, Grima!Robin has only two big weakness: Awakened Falchion (as it is a weapon of Naga made to seal dragons) and him/herself. Evil Delsin has plenty of powerful abilities, but a great majority of them are mitigated or negated by Dragonskin. Risen soldiers would hound Delsin in both air and ground, preventing him from swapping powers or at the very least damage him whenever switching. Hawkeye will ensure that whenever Grima launches an attack, he does not miss, and an Ignis-enhanced attack, backed by the skill Hawkeye, will massively damage Delsin, especially when Rightful God allows him to fling those attacks even more. Finally, when needed, Grima!Robin can just switch to tomes and swords, which could be just as devestating in battle as his breath (Nosferatu, Thoron, Arcfire as examples).
Delsin's best bet is to end the fight quickly or ambush him, but as this is Grima!Robin, his opponent will not hesitate to raze New York City just to see his opponent. High defense, resistance, and HP, combined with Dragonskin, will allow Grima to shrug off most attacks, and an undead army that he can call on demand (which includes healer, btw) will greatly limit Delsin's kill options. Any attack from Grima WILL hit, and as time passes, Grima!Robin eventually will figure out Delsin, which will prove fatal to him. Grima!Robin with medium to medium-hard difficulty.