I would also note that if you have a mixture of enemies in a swarm, they need to be slain in the following order:
Standard GS/Tyranid -> Adrenal -> Brood Lord -> Death Leaper
So you cannot kill a Brood Lord if others are in the swarm.
Not quite - it's:
Standard GS/Standard Tyranid/Adrenal GS -> Elite Tyranid -> Brood Lord -> Death Leaper
You can (and probably should) take out Adrenal GS first.