I've seen Slayer 3 times, and if you haven't seen them yet, you're in for a treat. They're so tight live, especially with the addition of Gary Holt years ago. Always a good time when I see them. Be sure to throw yourself into the mosh pit a couple times.

EDIT:
Slayer, Suicidal Tendencies, Exodus @ The Shrine Auditorium in Billings, MT 2014
Slayer, Anthrax, Death Angel @ The Wilma Theatre in Missoula, MT 2016 (Incomplete)
Slayer, Lamb of God, Behemoth @ The Kettlehouse Amphitheater in Bonner, MT 2017