No important trainers use OHKO moves, though some random trainers in late-game do.
Most bosses do not use mono-type teams. Gym Leaders and Elite Four members have one wild card.
Evil team members, Wallace, and Steven use whatever they want.
Enemy trainers do have EVs, but generally have subpar IVs.
Route 103 Rival: 5
Roxanne: 14, 14, 15
Brawly: 18, 18, 19
Slateport Museum: 17, 17 / 17, 17 / 18, 18, 20
Route 110 Rival: 20, 20, 20, 21
Winstrate Family: 22, 22 / 22, 22 / 22, 22 / 22, 22
Wattson: 23, 23, 24, 25
Mt. Chimney: 26, 26 / 26, 26, / 28, 28, 28, 29
Flannery: 32, 32, 32, 33
Norman: 33, 34, 34, 35
Weather Institute: 38, 38, 38, 39
Route 119 Rival: 39, 39, 39, 39, 39, 40
Winona: 40, 41, 41, 41, 42
Magma Hideout Admin 1: 43, 44, 44, 45
Magma Hideout Admin 2: 44, 44, 44, 44, 46
Liza: 47, 48, 48, 48, 50
Seafloor Cavern: 49, 49 / 50, 50, 50 / 51, 51, 51, 52, 52
Groudon: 55
Wallace: 51, 51, 51, 51, 52, 53
Victory Road Wally: 53, 53, 53, 53, 53, 55
Sidney: 55, 55, 55, 55, 58
Phoebe: 55, 55, 56, 56, 58
Glacia: 56, 56, 56, 56, 59
Drake: 56, 56, 57, 57, 59
Steven: 57, 58, 58, 58, 59, 60
Lilycove Superboss: Matches your highest, won’t go below 50