I need help on fallout 3?

im doing the main quest towards the end. the part where you went into the pipes and have to unclog something. when you turn the pipe thing and exit out the other door im greeted by a guy inn armor with a plasma blaster. how do you beat him with barely any health packs and with a little bit of ammo on my hunting rifle. btw where do you find lyon to give you the ok to wear power armor