you can beat the first 99lv heatran then bring a whole team to catch the second 30lv heatran
here is what I did it easier and safer :
transfer all your money to your alt account
use whatever poke which can 1 hit beat rocks like graveler and easily deal with golbat
at first let your poke to hold smoke ball
1 hit on a wild rock to see if it's sturdy or not
Catch by using ultra ball if it's sturdy
then train your poke by beating wild pokes until at least 80lv and catch any sturdy rock you see until full team
at this moment, you are already impossible to lose in a battle
then start beating npcs
and also beat the 99lv heatran
after that, talk to the npc in heatran room to get back all the master balls and rare candies
bring a whole team with you from pc computer, back to the cave to catch the 30lv heatran
npc won't challenge you anymore
good luck