Well bus seems to be the better option. Its just a 5/6 hour bus journey. The Haripriya to GTL and then wait for DEMU seems long.
1. A bus to Ballari and then lots of buses to Guntakal and then again lot of buses to Kurnool. That is the best option where no. of buses is more.
2. Shorter is - There are buses from Ballari to Kurnool direct too. That seems the best option in terms of journey time.