Here are your options.
1) Throw the Nuka Cola drinks you've collected at it while [censored] your only pants.
2) Pull a grenade on it's ass and hope he doesn't run towards you, missing the explosion intirelly.
3) Use VATS on it's hardened skin, injuring it but not taking it down.
4) Push your companion infront of you and run like the [censored] you're mother has always called you.
5) Pull your CZ57 Avenger out, put on a party hat and let rip.