There is a cancer eating away at the Imperium. With each decade it advances deeper, leaving drained, dead worlds in its wake. This horror, this abomination, has thought and purpose which functions on an unimaginable, galactic scale and all we can do is try to stop the swarms of bio-engineered monsters it unleashes upon us almost by instinct.
The best Tyranid attacks coordinate waves of small-but-vicious Hormagaunt broods racing across the table to attack while being backed up by larger and more powerful hive creatures like Tyranid warriors. Lictors can infiltrate and wreak havoc. Flights of Gargoyles use their bat-like wings to enter the fray. Monstrosities like the Hive Tyrant or Carnifex take out the toughest defenders. The Tyranids attack like a swarm of insects, sowing.