Get a shroomish to evolve onto a Breeloom at level 23- it learns Mach punch which is super effective
If you chose a Mudkip for a starter- Then that's easy use mud shot I hope its at least at level 25....
(The magneton doesn't have leviate)
If you chose Torchic-Double kick and ember will do nicely
If you chose Treecko- Weeeelll that's a bit more tricky. I don't think I used Grovyle for that gym at all when I played Emerald. But you could level it up to Level 29 and teach it leaf blade-until either of them dies-Yes Grovyle isn't much help for this one.
Catch a Geodude in Granite Cave and level it up to lv 18 for Magnitude. bring a few potions handy just in case it uses Sonicboom.
P.S - You probably do but SAVE BEFORE you battle him
