tazz, the best way to fight an experienced dynamo roller is to take them by surprise. it takes 47 frames (splatoon runs at 60 frames per second) for a dynamo roller to flick its ink and score a kill, so if you can catch them off-guard, you've effectively splatted them more often than not. while catching a dynamo off-guard may seem to be a simple task on paper, this is a bit more difficult in practice thanks to the incredible spatial awareness a good dynamo user must have to use the dynamo effectively and the possibility of them already activating their echolocator.
another way you could take a dynamo roller user on is to blitz and take them on in close combat; in this case, their only possible recourse is to run away or throw a splat bomb at their feet and try to take you down with them. alternatively, if you have a long-range special built up, you can use it to either splat them at a distance (inkzooka, bomb rush) or flush them out of their nests (killer wail, inkstrike). the close-range invincibility specials (bubbler, kraken) work if you'd prefer blitzing the dynamo since they can only run away in response. just make sure to have your special meter filled if you attempt this approach (and watch for pesky grates if you're using the kraken).
certain sub weapons are effective at neutralizing dynamos. at the forefront of these sub weapons is the disruptor, which heavily reduces movement speed and effectively removes the ability to jump for ~4 seconds. after throwing a disruptor, it becomes much easier to blitz and score a splat against a dynamo user. if you have good aim, you can also throw a splat bomb at the feet of a jumping dynamo. by the time they land, it should have already exploded, taking them down in the process.
chargers (minus the squiffers) and jet squelchers have better range than the dynamo, so if you're good at aiming and keeping your distance, you can shoot a dynamo down without having to worry about the almost-literal tidal wave that comes out from their flick.
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against noobs using the dynamo like they would a splat or carbon roller (i.e.: just rolling without abusing the flick), just back off and shoot them. they are easy prey for anyone who is semi-competent at the game.