Kit and Oldblood with the following:
Light armour, charmed shield, cold one, dawnstone, charmed shield, potion of strenght and piranha blade.
Here's the thing - The bloated fvck have no armour save and only a 5+ ward and 4 wounds. He's also only WS5 so the OB will hit on 3+, wound on 2+ and then there's the 5+ ward. Piranha blade will on average double the wounds.
So the OB will do:
5,8 attacks -> 3,8 hits -> 3,2 wounds -> 2,1 after ward. This will on average turn into 4 wounds. Bam. Dead. Worst case only 2 lol.
Put him in a unit of Cold One Riders and include a champion. If you just solo the OB you'll be bothered by a champ so you'll need one of yourself to counter that.
Afterwards you'll be faced with attacks from 6 plague bearers. The OB himself will 0,025 wounds by those due to the 1+ rerollable armour save.
**EDIT**
Do remember that he's a counted as a monstrous cavalry model. Unless put with other similar models he'll be left alone. It'll take roughly 45 shots (if we ONLY count the poison) to kill him. 10 cameleon skinks can put out 20 shots a turn.
Same can basically be applied to a great unclean one.
Don't be afraid of Epidermus' attacks. He'll only do 0,081 wounds to you. I haven't counted the 6 WS3 S3 nurgling attacks.
Until this is done try and not lose any models. Epidmus is only a sack of sh1t of you let him get 7+ wounds. Fortunately a nurgle army doesn't really have any ranged units and even if he brings a skillcanon it doesn't count towards the wound counter.
If you want to you can have a friendly Scar Vet with your OB. The SC can carry the other trickster shard that'll force rerolls on wards. Just put those next to each other and hope they end up in B2B contact with fatty.
What else in his army is troubling you?