The trouble with the Johto games is their notoriously awful level curve, making it difficult to prepare for the Elite 4 without significant grinding. Your best bet is training in Victory Road, as it contains the highest levelled wild Pokémon up until that point. Alternatively, you can grind levels in the Daycare, but that may take even longer than Victory Road grinding.
If you want to lighten the load, you can make cuts to your team. Magmar, while decent, doesn’t contribute much during the Elite Four battles due to the limited number of Fire-weak opponents. Magneton has its strengths, but compared to your other options, it may be the least impactful in the final stretch.
Edit: sumwun also says Rhydon isn't good for the Elite 4, so you can consider getting rid of that as well.
Hope I helped!