Doing HeartGold with only 1 Mareep sometime, and what should I do against jasmine's Steelix (maybe even her Magnemites will be a challenge)? I need help, plz
EV train. It'll take a while, but you can reach your Mareep's maximum potential this way, allowing it to take hits and deliver powerful blows back.
Overlevel. Grind levels somewhere (the Daycare is good), and steamroll her. This will take a very long time, however.
Double Team. Raise your evasiveness, and pray you don't get hit.
Swagger. Swagger's confusion can make Jasmine's Pokemon do major damage to themselves, but it's quite unreliable.
Hidden Power. You can hope your Mareep has Hidden Power Ground, Fire, or Fighting. If it does, you can demolish her with relative ease.
Hope I helped!