I haven't done this, but the short answer is, it looks easy. There are 1-2 buses a day. Check out the government's Taroko park website: www.taroko.gov.tw. It has English pages on bus transportation. The only problem with the bus is that that it doesn't stop at the oo-aa sites where you could get out and look more closely at marble cliffs and milky river and waterfalls, etc. If you wanted a quick jaunt up and down, the bus might work. But you could still consider a 1-day car rental from the station. Hualien-Taroko is not that difficult (or long) a drive, if you planned it out ahead of time. The terror-driving is mostly in the greater Taipei area. As always, try to avoid tourist spots on weekends.