Another way, join the enemy faction , then wait some days for regain your rep to -200 (or raise from -200 to 200 with the usual methods)
then leave the faction (work for Ncr -> raider, Unity -> vc or enclave)
you will become a friend
(you may join to other friendly faction then for raise up some more , for Ncr join Bos or Vc,
and you will be liked and then you will kill some noob with guards smiling to you

)