DeathGuard have a number of abilities, mainly centered around typhus and mortarion, that things in close combat with them, or with a DG unit within 7" of them, take mortal wounds on a 4+. As such, any protracted close combats tend to get costly for the enemy. Especially when you factor in disgustingly resilient....
As for me, all my games with nids in 8th (I've gotten about 6 in now) have been Turn 1, run stealers 30+ inches across the board to slam into target(s). Turn 2, run the remainder of the unit, typically 15 or so stelaers 30+ inches to other side of board to wipe other half of the army. Shake opponent's hand as they concede.
In a larger game, a single unit of 20 wont be as devastating, but the definitely will be still.
If you can, I'd try and slam that unit of stealers full strength into Typhus or mortarion first turn. Eliminate them and a good bit of the synergy of the army goes away. And don't be afraid to chuck them at vehicles. 80 attacks is nothing to sneeze at. Swarmlord is a beat stick in CC too. Had him charge by a nemesis dreadknight grand master last night - weathered the shooting and the charge, swung back and one popped the over sized marine.
I'll be interested to see how your flying tyrant does - not 100% on winged gun tyrants this edition... but then my Flyrant has made it to CC exactly once so far.
Also, avoid CC with the plauge walkers if you didn't already know that... mortal wounds plus they get to bring back models for ones they kill of yours makes for a rough fight. Shoot them up.