[SGMAlice]

Massed Bolter fire and Flame/Template weapons usaually do the trick.
10 Man Tactical Squads with Flamer/Missile Launcher
Predator
Vindicator
Throw some Assault Marines at them if they manage to get through the wall of fire. Try to get the charge on them too as it can get messy if you don't.
Its going to be messy anyway but you may want to limit that...
Depending on placement you may want to 'Remove' the Hive Tyrant first. Putting TL LasCannon on a Pred helps. Save the Vindicator for the Horde's. The Doom of Malantai, if used properly, will be one hell of a Ba$£*rd to bring down, so attempt to take him out early on too.

SGMAlice

 

[SGMAlice]

I agree with everything maybe except getting the charge. Most of his dangerous stuff is going to go at I5 or 6, so getting the charge, they are still going to go first with 2 rending attacks. Sometimes it is better to let him charge you into cover, because very few of his creatures have an "assault grenade" equivalent. (unless you are BA with furious charge, then you will get to go first against some things)
disclaimer: this is based on experience in games I have had against nids, and I have no idea what upgrades they had (although I know some of them had furious charge), as I have only skimmed their codex.

My SM army is also BA so i probably went from their Codex.
They only have I5/6 and possibly Furious Charge with !?adreanal glands?! I think thats what they are called, A biomorph (WarGear).
Recently had a List building session with one of my friends regarding whether or not to use them (He should have taken a Flyrant too but he was being stubborn)

SGMAlice