Dante could just Royal Block all of Akuma's attacks. Since Dante can easily react to bullets without a time altering ability his reaction time must be beyond bullet speed. Unless Akuma is proven to attack faster than Mach 1.7 (roughly the speed of how fast the bullets fired from Nero's Blue Rose are going) then Dante is easily going to see it coming.
Royal Block blocks all possible damage from Dante and stores it as Angry Energy which then can be released back at a target for equivalent damage plus a little more. With Royal Release Dante counter attacks by passing through an enemy, while the enemy is attacking Dante, and releases all the Angry Energy into that target while receiving none of the damage from the enemy's attack.
With perfect timing Dante is impossible to hurt while he is using the Royal Guard style and the boss fights against him show that he has this timing even when he is not trying very hard at all.
Among the things that have been Royal Blocked (which is every attack in the games that have the style Royal Guard) are: a sword that is said to cut through anything including dimensions, explosives, holy rays of light, lasers, blades, bullets, an exploding bullet that is already lodged inside Dante's gut (that one surprised me), punches from the Savior, and possibly a ray beam from Mundus (one of the most powerful beings in the DMC games).
All the things that can be Royal Blocked can also be Royal Released however Dante will only travel a short distance (maybe 15 feet) during the attack and will only harm enemies within that distance. The Savior's holy beam attack cannot be royal released (to my knowledge) because it is a continuous attack.