～ますまい is grammatical, but it's already fairly uncommon. It's mainly heard in role languages for stereotypical samurai and pompous/noble elderly people.
～ですまい is ungrammatical, although ～ではありますまい is okay.
Basically if you used ある for a person, it would sound more or less archaic.
～くありましょう is grammatical, but it's fairly uncommon and ...