Main Character Mechanics

 Before we get into the placement of enemies and the layout of levels, we have to settle on the abilities of the main character.

I think we could map blocking to a series of buttons rather than one, that way we have more buttons for attack and movement. I think jump could also be played with considering you attack more than you jump but I still think jump is a primary movement. The basic attacks are action game basics but the combos will make our game special. Here is my attempt:

Starting at the controls of the guitar:
Strum - movement left/right; attack left or right
1st button - jump (high enough to get onto platforms and over gaps)
2nd button - quick melee attack 

3rd button - heavy melee attack
4th button - ranged attack (weaker damage, can be blocked by shields, slow rate of fire)

5th button - Use/activate

hold 2-4 down  - block

Raise guitar - aim up
Lower guitar - aim down

Whamy bar - Use the whammy bar like you do in guitar hero for lengthy attacks. For example: you ram your guitar into the heavy enemy, you then wag the whammy bar to increase damage. 

The way I imagine weapons is that the melee won't change much outside of color and the guitar in the players hands. The ranged attacks are different depending on the guitar. The player unlocks a new guitar per world that they beat. They can then replay the game with the new guitar for different gameplay.
Weapons:
Metal guitar - basic attack
Shotgun Banjo - shotgun blast
Funky Bass - heavy attack
Keytar/synthesizer - Laser rifle

Unit Tactics:
Weak Unit - Any weapon attack works. Bigger threats should be tackled earlier
Ranged Unit high up - Ranged Unit can stun player and then player can get overrun by weaker units. Player has to get under the sniper and kill him quickly
Equal Unit - Combo attacks from the front. If player attacks quickly enough, block can be broken down and they can kill this unit. Weaker units should be killed first so player can focus on Equal.
Shielded Unit - Shield unit can't be killed from the front unless the player uses a special ability and it's earlier on. Player
Heavy Unit - Player must wait until cool down from heavy units attack for critical damage. Or use special weapon or ability to weaken.
Bomb/Suicide Unit - Ranged attack to blow up bomb before unit gets to player
Special Unit/Boss unit- require special strategy to kill