I was doing the mission for the Brotherhood of Steel in which you kill the railroad, Deacon used to be my companion, and when I got the orders to eliminate the railroad, when I needed to kill Deacon, he would take no damage from me, and I can not complete the mission otherwise. Please help! I'm almost have finished the game!

Use his recall code

@vmag I tried! The paper said "You can't trust everybody" and just so you know that is not his recall code!

Simply dismiss him, then kill him.

@No.64_Urizen I cannot, as soon as I said yes to the mission, he was instantly dismissed, and he can do damage to me but I cannot to him.

I've heard of this one and someone else having the same issue; the minute he's a companion he's unkillable/essential, and joining the brotherhood doesn't change that. It's a pretty serious bug.

If you are on a PC you could perhaps use the console to kill him/mark him as non essential.

If you are on a console you may be screwed

@Kalyana That ain't good to hear because I play PS4...

Are you on the PC? If so http://fallout.wikia.com/wiki/Fallout_4_console_commands to set him to killable -

• setessential – Allows an NPC to be permanently killed. Click on a an NPC then enter the command for it to work. Use the resurrect command to revive them.
  • setessential 0 – Allows the NPC to be permanently killed.
  • setessential 1 – Sets them to be unkillable.

Already posted a solution if you were on the PC...sorry for the tease :/

@Tawnee I'm on PS4

P.S. I am responding to all of these because I'm eager for a solution

Your best bet now is to revert to an earlier save, and dismiss Deacon well before you take the kill Railroad mission.

I'm sorry man, but I'm not sure I am aware of one; until they patch it, Deacon is unkillable, and yet your main quest is requiring that you kill him.

The only solution I can suggest is to ignore the main storyline, go off and do something else, and hope that they fix it in the next patch.

Ok, I read the latest patch yet there was nothing including this, and can't go for the railroad ending, because there all dead now, Institute ending has me kill the railroad aswell, and the Minutemen ending in a bit boring. Thanks for the help everyone, all I can do is hope for it in the next patch.

No way Im doing that, I had Deacon before I even joined the Brotherhood

No just biff him before taking that one quest.

I said that I had Deacon before I joined the Brotherhood, and that's like a hundred saves ago.

So you took that one quest to kill the Railroad a long time ago and did a ton of things inbetween?

No, Deacon was my companion well before I joined the Brotherhood, much later I got the quest.

Yes good, then revert to an earlier save JUST BEFORE YOU TAKE THAT QUEST, dismiss Deacon first, then take the quest.

I'll try that, but if not thanks for the help anyway.

It's your best shot anyway. Maybe having Deacon with you at the exact moment you took the quest, and having the game reassign his faction is how you got bugged out...maybe game screwed his faction up for you.