Militant libertarianism isn't an oxymoron because libertarians don't say "do no harm" they say "do not initiate force". It is impossible to not initiate force unless you are in line with libertarianism, and thus from a libertarian point of view it is not immoral to respond to non-libertarians with force until they either are incapacitated (they can be against libertarianism, they just cannot act on this, as to act on being against libertarianism is by definition to initiate force), dead, or libertarians.