((Not a teacher))
Welcome to the forum.
"Mess with the best, die like the rest" means that the "strongest person" (best) would simply kill (through violence like shooting) the other person if he/she insulted or offended him and the opponent will die like the rest of the people, who dared to challenge the best.
I guess it is usually used in the military.
Hope it clarifies.
Student or Learner