Fun mix. Assuming (1) the Murder's a Murder and not swappable to a Carnage, (2) following the 2010 FAQ and using the Powers of Chaos Nurgle fleet list, I'd offer the following options at a few points levels:
750pts:
Hades (200)
Murder (170)
Slaughter (165)
Devastation (190)
Chaos Lord (Ld +1) (25)
- Closing you've got 6 lances between the Murder and Hades. Squadroned it's an easy Lock On check to start working on crippling a cruiser before presenting a broadside, and a FP 20 & 2 Lance broadside (alternately FP 14 & 4 Lances) will do most of the rest, while the Slaughter flanks. Devastation provides extra broadside lances from a slightly trailing position and aircraft as needed. Lord on the Devastation or the Murder depending upon personal preference.
1000pts:
Hades (200)
Murder (170)
Slaughter (165)
Devastation (190)
Chaos Lord (Ld +1) (25)
Infidels x3 (120)
Infidels x3 (120)
- Same plan, but you can synergize torpedoes and attack craft with one Infidel squadron as you close, while the other squadron helps cover the Slaughter. Alternatively, field 4 Infidels in one squadron, and pair 2 Idolators to reinforce the Slaughter with extra lances on its attack run.
1500pts:
Despoiler (400)
Repulsive (+1 shield) (245)
Hades (200)
Murder (170)
Slaughter (165)
Devastation (190)
Warmaster (Ld +2) (100)
Chaos Lord (Ld +1) (25)
- Warmaster takes the Despoiler, start the Slaughter on one flank of the Despoiler with the rest of the fleet on the other side; it'll draw attention away from the Slaughter and set you up for a turn 2/3 wide flank into your opponent's rear arc. Here the Repulsive offers torpedo/attack craft synergy, while providing almost as much firepower as the Murder and Hades combined in its broadsides.
You'll at least have parity in attack craft generation, if not outnumber your opponent.
- - -
These are just a few introductory take-all-comers fleet options based on my assumptions above. Not knowing your level of experience with BFG I went light on escorts since cruisers are more forgiving. I realize I didn't Mark any of these ships in this fleet build, but at +35pts, the Mark of Nurgle's cost exceeds its utility (the BFG:R fleet lists fix this), and Demonships are even more out of whack regarding cost vs. effectiveness in the Armada rules.
The Carnage's single-target broadside capability is brutal, but Murder/Hades pairings can carve through Armor 6 prows where a Carnage will have to work a little harder.
Don't forget you have boarding torpedoes and assault boats. A salvo of boarding torpedoes into a cruiser's flank from the Infidels can shut down its broadsides with critical damage, letting you get free punches on it for a turn or two before your opponent can repair the damage.
Under the BFG:R rules for Chaos ships, I'd consider Marking the Repulsive to push its damage-soaking capacity up even more, and possibly adding Hives of Nurgle to it then. You'd pretty much force your opponent to have to deal with it at that point, which would pull some heat off of other ships (particularly the Despoiler, which practically screams "Shoot Me!"). If you wanted to Demonize something, use the Slaughter as a lurking threat until your fleet is crossing the 45cm broadside gun range, then pop her out of the warp wherever you want her to pour fire into something from the opposite side once the Repulsive or Murder/Hades combo has engaged a ship, torn off its shields and started to bleed it. To bring in some if these upgrades you could probably safely drop the Devastation at 1500pts, buy the upgrades you want, and fill out the rest of the points deficit with Infidels.
These are just some options; there's a bunch of other possible builds in there, and I'm sure you'll get some more useful suggestions.