Van may be a master manipulator at heart, but he's no slouch at face to face combat, and what better way to prove that than by slaying a dragon? Van may not have the early punch to stop Ryu from transforming, but that's not a problem. This Ryu lacks the earth shattering power that the second and third incarnations have, so he'll need quite a few breaths to take down Van - and that means Van will be able to get off multiple mystic artes, which are an assault not even a dragon can withstand. Ryu lacks the stamina to transform multiple times into his most powerful forms and still attack a lot, so eventually he'll be caught in human form, then dismembered in sword to sword combat like the inferior being he is.