I as a commando did it by initiating heroic moment and doing as much damage as I could before Tanno died, then I used cryo grenade and ran (by the way, it helps if you have the whole story area cleared of Imperials before doing this) around the ship, he takes a while to catch up which I used to heal myself with medical probe and medpacks and adrenaline rush. Then I waited for him to come, and did a lot of damage again , used cryo genade and ran again and healed myself and on the final round he fell. If you are a Vanguard however I would reccomend using guard on Tanno Vik and use the ability that makes the enemy attack you for 6 seconds when Tanno is sustaining heavy damage so you two can split the damage and then you can use cryo grenade when Tanno dies and heal yourself with a medpack and adrenaline rush and you should have done enough damage by then that he will fall easily.