Charger-type weapons generally are not used to cover as much turf as the Shooter or Roller weapon types. Instead, they provide cover and/or pressure to high traffic areas of the map, as well as key choke points, allowing the rest of the team to move more freely.
The Kelp Splatterscope has the same set as the Kelp Splat Charger, so the general strategy will be the same here - pick people off from vantage points, while covering turf with your sub and using the Killer Wail to force people from hiding or keep them stationary.
Tips for facing each weapon type
Shooter-type weapons will give more trouble than you would have with another Charger set, because you lack a tactical sub weapon. Instead, be extra careful to abuse your range and not let these players close. If you push them behind a wall or into a tight space they might get the advantage over you, just back off, or find a different vantage point if they're threatening you. Be careful not to stay scoped in too long, or they may move to your blind spot.
Roller-type weapons are the easiest targets to pick off for Chargers, aside from the Inkbrush. Taking out these players will be your priority, and you can throw a Sprinkler over the area they covered to let you get back to picking off other players, instead of covering where they went.
Charger-type weapon matchups are difficult, and is mainly about getting a sneaky shot in while they're not paying enough attention. The scope weapons are at a slight disadvantage, because while they're scoped in they can't see the targeting beam from enemy aiming Chargers, so you'll need to be extra careful. Another option is to ignore the enemy Charger (so long as you don't let yourself get sniped), and deal with the rest of his team. A lone Charger will find it very difficult to fend off an entire Splatoon!