You have to be restrained and wait to use your rush until they're as close as they get, then rush a couple ships all at once towards the centre of their fleet/their biggest ship and board it as many times as possible.
Other methods are cheap escorts with the tentacles, but instead of using the one that eats troops, use the use which pulls the enemy closer.
The sporemine cloud is good as it slows down enemies and you can hide in it.
Stealth you ships as much as possible so the have to get close to spot you again.
And ram if possible.
Hey, I've been on both sides of this and it is really overpowered. I went from level 3 to 11 as Tyranids without losing a single game using: 1 infestation hive ship (the x7 carrier), 4 of the infestation (x4 carrier) battle cruisers, and an escort or two.
I am not that good a player, Tyranid hangers are too strong, however I have actually managed to beat it once (with a balanced fleet as well), it was very close and I knew exactly how to counter it from my experience with it.
The battlecruisers ignoring morale upgrade is too much for how good the carrier battlecruisers are.
But essentially everything else is too weak, I'm not sure I've ever lost to a non carrier Tyranid fleet, I think for the launch, Tyranid hangers should be way weaker, but scuttle damage should be too (Tyranids are forced to fight close unless abusing hangers), and skeleton crew penalties should be increased, so factions that rely on close range boarding actually have a chance.