User blog comment:B214/Volume 22 spoilers/@comment-28087053-20170114184806/@comment-29963364-20170115074635

Indra's power is on par with the current Four Great Satans combined (assuming Sirzechs and Ajuka don't go all out). But what makes Indra so strong isn't how powerful he is, but that he's a skilled warrior.

The Hindu Mythology is regarded as the strongest among all the Factions, because almost every God among them is an experienced, battle-hardened warrior.

Take Sun Wukong for example. He admitted in vol.17 that he's not strong enough to take a hit from Issei or Vali, but they still can't beat him because he out-classes them as fighters. Similar to Cao Cao, who is a simple human, who uses his combat skills to fight on par with the strongest of beings.

So in terms of power, I'd say Indra is below the Heavenly Dragons. But his battle prowess would easily put him on par, or above them.