For it to oneshot you have to aim it to the front-downwards so it won´t reach max range, at max range it doesn´t one shot. So if you stay at max range, you can even take one hit and still have time to approach the Dynamo user and kill him before he does another flick. Also, the Dynamo user has to jump to get to that range so that makes him even more vulnerable. Just don´t approach it head-on and take advantage of your superiority in speed and agility, the same way you would you when assaulting a good charger.
Also, you can hug the Dynamo and the ink will go above you even at point blank distance.
Another thing is that once a Dynamo fires once, everyone in the vecinity will know exactly where he is located. Sneaking around a defended position in a fast manner is almost impossible.
Moving around with it over unpainted territory is very slow, painting even the smallest patch of terrain around you makes you vulnerable for a long time and consumes a lot of ink...Maneuvering in small places (ie; the tower in TC), or places filled with irregular geometry (like the splatzones in Moray Tower) is difficult too.
The weapon has blatant weaknesses, you just have to use the ways to exploit them instead of trying to get into an exchange.