In fact, Lady Shiva was the person who taught Batman how to fight again after he had his back broken by Bane.
For one thing, Lady Shiva is a huge character in DC Comics.
On a show that has already brought in the likes of Deathstroke and Ra’s al Ghul, a big character from comic lore would make sense.
An assassin-for-hire, she is regarded by many within the DC Comics universe as one of the best living fighters, using her bare hands and finely honed talents to take out her targets.