strong wallbreakers such as Specs Pyroar and Specs Aurorus can break through that core without too much trouble. Both of them can be killed by Steelix so you'd have to break the sturdy first though.
CM + Psyshock + Heat Wave Xatu can also break through the core, although Sludge Bomb poisons are really annoying, and it has to roost up on Steelix quite often.
Thanks!To be fair you could do pretty well just running a faster Weezing of your own with Taunt. It stops Steelix outright, can lock Weezing into Sludge Bomb, making it much less threatening, and stop Audino from using Calm Mind or Wish and chip away with Sludge Bomb if it's already Mega Evolved. I sometimes run 252 HP / 92 Def / 164 Spe Bold, which also outspeeds Jolly Rhydon.