Jasmine's Steel-type Pokemon have a lot of resistances, but they have three pretty big
weaknesses: Fighting, Ground, and Fire. If you still have that TM28 (Dig) from the
National Park, you should see if you can teach it to one of your Pokemon (if you haven't
already), because that will be very effective agaisnt Jasmine's Magnemites. It will also
be fairly effective against Steelix, though its ridiculously high Defense will still
require you to hit it two or three times to KO it for sure.
A strong Water-type Pokemon with Surf should be able to take Steelix out without any
problem. It's the main threat on Jasmine's team. It knows Sandstorm, Iron Tail, Rock
Throw, and presumably Earthquake, making it quite aggressive in nature. It also has a
Sitrus Berry which it will consume when its health is low, restoring 25% of its HP.
After beating Jasmine, she'll give oyu the Mineral Badge, when prevents traded Pokemon up
to level 70 from disobeying you. She'll also give you TM23 (Iron Tail), which is a
powerful  though rather inaccurate  Steel-type move.
Now that you've beaten Jasmine, it's time for you to Fly (or walk) back to Ecruteak City
and head east past Route 42 and over to Mahogany Town!