There are many characters in Naraka: Bladepoint and while they have the same statistics, they do feature different abilities. This is especially true for their ultimate abilities. Some can render you invisible, heal yourself and your allies, or summons a whirlwind of sand.
But only one character lets you transform into a towering Vajra and that is Tianhai. His ultimate ability, the Titan's Call, transforms Tianhai into the massive Vajra. When he takes on the six-armed form it can be incredibly difficult to deal with his ultimate, but there are a couple of ways to do so.
The first and most obvious option is to run. The Vajra can neither be transformed into and enter buildings as the space is too small for the Titan. Players can also hide behind rocks, trees, and other objects to block their ranged grab attempts.
To fight the Vajra, your best bet is to put some space between you and the Titan. This allows you to equip a ranged weapon and target the Vajra's weak spots. A random body part is chosen and it will glow to signify which part it is. Targeting it with ranged attacks will eventually fatigue Tianhai and the Vajra will drop to one knee leaving him completely vulnerable. It takes about four to six shots depending on which weapon you have equipped to drop the Titan into a fatigued state.
For more information on Naraka: Bladepoint, check out How to counter in Naraka: Bladepoint and How to use the grappling hook for combat in Naraka: Bladepoint on Pro Game Guides.