Well, lets look at miltank.
So 3 stats all around 100 at a point of the game where you may luck out with a highest base stat of 80 (bar an Onix)
Only 1 weakness in fighting, which at the point of the game you probably have maybe 1-2 moves to choose from, Rock Smash, which with only base 40 vs Miltanks 95/105 physical bulk will do virtually nothing, and maybe a double kick Nidoran/Nidorina/Nidorino if you went ahead to catch it. Which is OKbut Miltank has ways around this
Basically, this thing is an NU Haxgod in competitive.
Milk Drink + Super Potions = Tons of healing.
Attract, generally when you dont have SE moves you usually go to your highest Level Poke, which is usually your starter. starters have an 87.5% chance of being male. Miltank is always female. Happy valentines day.
Stomp: So you probably have 50% chance to attack from attract, let's use 100 base speed with a stab move and 30 chance to flinch fix that. Oh wait you hit me? Drinks Milk
Rollout. Let her hit 2 in a row, then pray to Arceus she misses one or your dunzo funzo.
Conclusion: Since it's practically impossible to one shot her(or even 50 her) she pretty much will hax you to death. High Stats Great Recovery and little weakness at this stage of the game makes her a menace.