I did it with a combo of Retribution and Armageddon/Overlord class ships. Cycle launching melta torps to chip it down and then straight up ganging up on it when it closes in won the battle fairly quickly for me. Make sure Spire is not the first one in and he should avoid most boarding attempts.
Alternatively a full fleet of SM ships with lock on and bombardment cannons. Kite away, and as soon as the shields are down bombers and boarding should chip it down pretty comfortably without putting yourself in much danger.
And since bombardment cannons have longer range and if you kite well, setting the Ancient as prio 1 will ensure that most of your broadside firepower is directed towards the smaller ships.
Maximum cheese seems to be the way to go. Ark Mechanicus with 4 Dictators provides a lot of Nova firepower and vision+harassment with fighters and bombers. You got enough range to keep the enemy at bay for a while and can trade somewhat with more longer range factions.
The problem is as stated above when you face a more in your face faction or if something gets to close. Low troop value and not great DPS depending on your setup can spell death for your fleet.
I think we all can agree that this is indeed a mayor bug. As soon as the first or second chunk drops to 0 the game somehow treats that as full crew death. Strangely this seems to only apply to the Tyranids as I have yet encountered it with any other faction.
The Cairn certainly does not feel that worth it when you look at its point cost and what you expect from it both lore and crunch wise.
What sort of saves it for me is starpulse spam where you just jump onto the enemys face with the upgraded star pulses and blast most of their fleet into the next match provided they are bunched up (still fairly effective against 1-2 ships aswell though).
Not only that, but I also had a weird encounter with the Necrons as the Tyranids, where boarding parties and Bio Acid weapons would strip kill off Necron crew seemingly at random.
The fight lasted only a minute as each boarding party and Bio Acid volley basically turned all the Necron ships into hulks.