Fighting Deathguard

Unfortunately speed, range and heavy weapons are the three things Sisters are short of compared to other armies. Attempting to put together a list based on this will just get you mangled.

Instead you need to concentrate overwhelmingly on the Sisters core strength, short-range shooting. Nurgle is good at this too, but not as good as the Sisters, he'll be counting on assault to tip the balance. You need to smash him up and avoid cc for as long as possible.

To do this take large groups of dismounted Battle Sisters and Retributors and deploy in a defensive formation. This is two lines, the first of the Battle Sisters armed with bolters and flamers, the second of quad HB 10-strong Retrib squads. Space all squads 7" plus apart to defeat consolidation assaults. Let him come to you, firing at him with all weapons all the way in. Concentrate your use of faith in DG on the Retribs, who may well benefit from an Imagifer to make sure of it. Remember that fearsome as the Death Guard toughness is, their armour save is actually still the greater part of their defense. If you negate that you triple your casualty rate. Don't worry about your T3, all your defensive power really resides in your armor, your numbers and in the last resort SotM. Every army in the game gets good to-wound rolls against the Sisters, but that counts for nothing if they lose 2/3 of their shots to your armor.

Don't be afraid to move the first line forward to get a better shot in as he closes. One good round of bolter/flamer shooting with DG could cripple his army. Once he hits your line in assault you need to make a decision on whether to tar pit him, abandon the Sisters to their fate or counterattack. Tar pitting uses SotM and LotE to keep the Sisters in the fight until they are very slowly wiped out. This can tie down his best assault unit for the rest of the game. Abandoning your Sisters means allowing them to get slaughtered so the rest of the army can resume shooting when his unit is left standing in the open. Make sure the rest of the Sisters are properly positioned for this. Finally a counterattack is when you bring in the Seraphim or Celestians led by the Canoness/es to carve him up.

In regard to your counterattack unit, whether you use Celestians or Seraphim, don't be afraid to launch a pre-emptive strike. This can stall his advance and allow you to make the decision as to when and where cc begins. The classic Sera tactic of Falling Back in his assault phase to leave his marines exposed before your guns should be exploited to the full.

With vehicles I'd say take it easy. You don't need transports and two Exorcists may struggle to find enough targets against an infantry based DG force. If he does take a lot of vehicles by all means field 2 Exo's if you have them. Immolators are of little use here as they cannot get the DG you need to have any chance of hurting the plague marines. A Celestian squad in a Rhino with a Canoness is a good counterattack unit, although I would prefer Seraphim.

You should burn faith like crazy for this game, especially on DG, so two canonesses are probably a good idea.

And finally if he refuses to come to you and tries to shoot it out at long range? In the unlikely event of this simply march slowly forwards as a mass in formation. Even the Retribs should move up if they need to keep in range. Eventually you'll pressure him into an assault by closing to short range. If he tries to shoot it out at close range, you'll beat him on attrition.