You will still unlock most of the unlockable gear if you ask for help in completing the levels in the Splatoon 2: Octo Expansion. This makes it possible for younger or less experienced players to unlock most of the exclusive gear available from this expansion pack.
You will need to beat each stage on your own to battle the secret boss. Defeating the secret boss gives you the "Golden Toothpick" headgear.
The gear you can unlock in the Octo Expansion is as follows:
Unlock Requirement - Item - Ability
Complete 1 line - Octoling Shades - Last-Ditch Effort
Complete 2 lines - Neo Octoling Armor - Haunt
Complete 3 lines - Neo Octoling Boots - Object Shredder
Complete 4 lines - Null Visor Replica - Special Power Up
Complete 5 lines - Null Armor Replica - Ink Resistance Up
Complete 6 lines - Null Boots Replica - Drop Roller
Complete 7 lines - Old-Timey Hat - Comeback
Complete 8 lines - Old-Timey Clothes - Thermal Ink
Complete 9 lines - Old-Timey Shoes - Run Speed Up
Complete all lines - Conductor Cap - Sub Power Up
Defeat secret boss - Golden Toothpick - Special Charge Up
Purchase Octo Expansion - Studio Octophones - Ink Recovery Up
Purchase Octo Expansion - Octo Layered LS - Ink Saver (Main)
The unlockable gear appears in a gift box in the center of Inkopolis Square the next time you visit upon completing the unlock requirements.
You can also complete the Octo Expansion story and unlock your Octoling character without even discovering all of the lines, as long as you find all four of the Thangs.