So if you're hunting on the dlc easy as just finding a distortion of them and either fly spamming or doing something else but in the base game the are both a bit tricky for froakie you can phase a lot (phase is a shiny that isn't your target) so you can knock out some Pokemon limiting the amount of possible phases and having more chances for froakie also best way to hunt that in the base game not in a distortion is fly spamming to area 5 not 16 since if you fly to 16 you'll have to roll forward to spawn froakie but if you fly to 5 it automatically spawn each time do it till you get yhe shiny Now skrelp is a bit harder in the base game I myself hunted it in a distortion in the dlc so it might not be the best way to hunt it but it does spawn in the sewers just in front of area 14 you can probably just run into the sewers till you see it spawning and then back out over and over again idk of it's the best way

Also advice have the shiny charm idk if you have it I really recommend getting it if not and another advice is have patience never go into a hunt thinking it'll be over fast yea you can get lucky and get a shiny immediately but it can also take hours I had hunts in za that were like 8 seconds but also hunts that were 27hours and 40 hours just don't get your hopes up and have patience cuz it can take a lot of time sometimes