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troperville

In Devil Bear , Bearalzebub rocks out on a guitar to defeat a teddy bear parody of Kiss.
Erfworld has a literal rock battle. Between Italian vampires and death metal midgets riding ~~dragons~~ dwagons.
- Uncroaked troops led by a Croakamancer can actually Dancefight.
- Apparently, rocking out is even more powerful than standard dance-fighting, since rock music is the music of the Titans. Makes sense, since the Titans are mile-high Elvis impersonators.
My Name Is Might Have Been is the story of The Power of Rock After the End.
- Inspired by Rock Band, of course. They cover pretty much everything. Character creation, DLC, buying new clothes, earning the tour van (and, by implication, the other vehicles), losing fans when you screw up, even Overdrive/Star Power is made into a major plot point.
Attack by German Composer in Not Quite Daily Comic.
- Seems to be inspired by Asuka's Mind Rape in Neon Genesis Evangelion.
An early Dresden Codak that's no longer on the site parodied this, by showing how well it would work in a realistic setting — i.e., not at all.
On the psychological warfare front Schlock Mercenary introduced the Burana Bots
Wonderella uses an electric guitar to subdue 500 of Hitlerella's angry bears in "The 500 BEARable Lightness of Being".
In The Challenges of Zona a somewhat NSFW computer rendered webcomic, Mentl, a struggling rock guitarist of slight and scrawny build, finds himself in a barbarian fantasy world where he almost immediately (and quite unknowingly) seduces the titular Zona, a swordfighter, by singing a Beatles song. It turns out that he has latent magical talent and can use the lyrics of his favorite songs as powerful spells. For example, he accidently places every woman in a ballroom under a love spell while singing The Troggs' "Love Is All Around", and flash-fries an attacker by belting out Jerry Lee Lewis' "Great Balls of Fire".
Keychain of Creation has the villain Resonance Ben who can, in his own words, "channel the rage of the Neverborn through my guitar strings," though it primarily harms his fellow denizens of the Underworld. Ben later reappears with a band made up of ghosts and undead monsters who use instrument weapons and magical music that harm the holy in general and Solars in particular.
In Sonichu, the 4-Cent Garbage building is demolished by, believe it or not, using Guitar Hero/Rock Revolution instruments to sing a song about how much better Power Rangers used to be, sung to the tune of "Cat Scratch Fever."
In Soul Symphony, the enemies in a metalhead's soul use this to battle, like strumming a power chord to shoot lightning bolts.
In Galactic Maximum, you need it to survive the Wretched Hive.
The Hook (by comic book vet Mike Baron) is set on the Granite Planet, where music has destructive power. Most of Earth's musicians fled there when music was outlawed, and now they're fighting it out among themselves. Baron cites the "battle guitars" in Ostrander's Grimjack (see above) as an inspiration.