User blog comment:B214/Volume 23 discussion blog/@comment-76.175.162.45-20170406042604/@comment-31137559-20170406183115

That's because Shiva is the strongest existence behind Great Red and Ophis. Indra could just be a battle maniac who views Shiva as the ultimate challenge. Like Vali wanting to fight Great Red one day. And Issei doesn't need any more teaching or training from anyone. What can Shiva teach him? Nothing really. Issei has all the power he needs. We're in the final stage of DxD so it's kind of too late for anyone to be training Issei.