The lead guitar player’s style is invincible, so it’s impossible to imagine how the band could ever replace him.

The word “invincible” in the context of your sentence means unbeatable or incapable of being overcome. It describes the lead guitar player’s style as so unique and powerful that it’s hard to conceive of anyone else taking his place in the band.

If you have more questions or need clarification on anything else, feel free to ask!