Ok this is a *spoiler for those who haven't gotten to the volcano level*.
This is just another way to kill Doyle and it allows you to avoid the elite guards. This could be a bug; however I get partial results everytime. Once you get to the top of the middle structure by way of the elevator, stop at the ramp that leads to where the guards and Doyle are. Since Doyle is up in the room above the guards; fire, preferably rockets, at the dark like windows. If Doyle is near the windows this will work. After awhile of firing, Doyle will talk to you. When he says "you are a resourceful man" that means he his hit. Keep firing and he will die eventually. I killed him this way 2 out of around 50 times. I believe it only works if he is near the windows. Like I said it could be a bug. However, everytime I have gotten him to say the one pharse, he is a bit bloody by the time I get up to him, so it probably means it does work.