Shaiya: If mobs have AoE, keep them away from other party members. Running in line back and forth is less effective than in circles, but allows mages to be at the max range all the time, making it safe for them, while still giving a bit of kiting advantages. Stop Blow is used to stop the mobs while tank goes out of their range safely for the time.
As Attacker: It's most likely impossible for a Battle-Tank to outdamage either fighter/warrior(shaiya money) or any offensively built caster during normal grinding. That said, your role would be to support the main tank or just act like while soloing. Unlike fighter/warrior, you have both the skills and defense to draw mobs that run stray onto yourself and probably even if the main tank dies, to keep all the mobs until they are finished.
If a mob runs, use stop blow if you have it, to give time to the main tank for recovering aggro. If the bunch is mixed, and you have a tank, your duo can really make mobs worthless. When the tank uses Darkness Blow, you may use Silence Blow, blocking all the mobs from attacking in one go, and keeping them so when you change the order. At level 55 with both Silence Blow and Darkness Blow at level(Shaiya powerleveling) 3, this really shines.