On second thought, Scion's main method of attack is his wavelength negation and dimensional travel.
Waves travel, they have a magnitude and direction and can even be expressed in the form of rotating vectors called tensors. I don't see why Accelerator would not reflect them.
His dimensional travel is also something not completely new to Accelerator. He can already reflect dimensional travel through the eleventh dimension and his crossover feats include manipulating vectors of parallel worlds when he was introduced to them. That's not enough to counter Accelerator.
The only problem here is his adaption, he can calculate counters to any phenomena he encounters nigh instantly. PTV also has vastly more calculative strength than any shown esper including Accelerator- Mina Mathers has shown having a greater calculative ability can allow you to work around Accelerator's own calculations for vector shield.
So Scion wins only if he has PTV activated. Otherwise, he would lose and can just helplessly retreat to other worlds to survive. (OBD counts that as loss, so there's that)
That's cap, Othinus slams. @Paxton you're taking the L here.