The trouble with the Johto games is that the level curve is just awful. It'll take you a while to level up at all, let alone enough to challenge the Elite 4. Your best bet is fighting the Pokemon in the Victory Road, the highest levelled wild Pokemon 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 actually dump a few team members. Magmar is fairly unnecessary for the Elite 4, with only a few Pokemon being weak to Fire. Magneton is quite good, but it's probably one of the least useful Pokemon on your team when it comes to the Elite 4.
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!