We actually do know the list of opposing trainers in Emerald. In the gym battle, Norman's Slaking is suffixed with a , which means it has IVs of straight 30 and, like every opposing Pokemon outside of the Battle Tower etc., no EVs anywhere.
The maximum possible Attack stat on a level 31 Machoke is 105. Because of badge boosts, having the Stone Badge means this will really count as 115, but even then, you have this:
252+ Atk Machoke Low Kick (100 BP) vs. 0 HP / 0 Def Slaking: 112-132 (78.3 - 92.3%) -- guaranteed 2HKO
...And Slaking has both Sitrus Berry and Counter for when you come up short. Don't bank on that.