some tips:
-use gyro controls
-start with the Squiffer or Kelp Charger
-go to the test range and find out how long you have to charge to deal 100 damage to the training dummies. Try to shoot all 3 dummies in the lower corridor with 100 or slightly above 100 damage in one go (or 6 dummies when the others respawn)
-equip Damage Up gear, something like 2 Damage Up main and 3 Damage up main and two subs, then do the dummy test again to see if you can do this faster
-when you can't directly aim at an enemy, assist your team by creating ink trails to key positions on the map, and cut off enemy escape routes so you trap them in one spot
-position yourself so have a good view of the area you're supposed to cover that can not be easily reached bgy any other weapon, and take advantage of your range. If an enemy comes to you, make sure you have a relatively safe way to escape so you can get out of their range of fire and reposition yourself
-for close combat fights, the Squiffer will teach you how to do duel others with quick precise sniping and mobility, if you have blast bombs or splat bomb, they work mostly a detraction while you prepare your escape