Had no real trouble with this mission
Actually my SM fleets suffered quite heavy looses
My IN fleet has quite a lot of plasma macro and torpedoes
As for every game vs Tyrannid I had all my ships with Lock On enabled and engagement distance set to the maximum possible
My ships were outside of the scream radius most of the time
The Ancient One was a priority 4 target. This way my ships would actually engage the true threat of the mission : the other tyranids ships
I spammed the Melta and vanilla torpedoes on the AO which is just a huge barely moving target. Used lighting strike whenever possible too on it
It finally died due to 0 crew left but it had only 300 hp left when it happened. The SM reinforcement came just to claim the glory as they barely got in range when the mission ended
I actually think would have lost less ships with my long range AM fleet