Nobody will end up dead

Question

In this battle you are choosing
weapons hard or sharp or flat,
sometimes winning, sometimes losing,
otherwise a futile threat.
Despite the arms which you are using
nobody will end up dead,
for this fight is just amusing,
exists solely in your head.

Answer 1

Surely it's

A game of rock, paper, scissors.

In this battle you are choosing weapons hard or sharp or flat,

Hard - rock; sharp - scissors; flat - paper

sometimes winning, sometimes losing, otherwise a futile threat.

Three equally likely outcomes

Despite the arms which you are using nobody will end up dead,

Rarely does someone become Violent

for this fight is just amusing, exists solely in your head.

Answer 2

Music?

In this battle you are choosing
weapons hard or sharp or flat,

Hard, sharp, and flat musical notes.

sometimes winning, sometimes losing,

Going up and down in pitch?

otherwise a futile threat.

Nobody is harmed.

Despite the arms which you are using

You use your arms/hands to play the piano, the violin, and most other musical instruments.

nobody will end up dead,

Obviously.

for this fight is just amusing,

It amuses your audience.

exists solely in your head.

That is, if you can play music without seeing the scores!