How can I deal with a Dragon?

For my next game, my opponent want to bring in the following character with a level 4 battle magic wizard and a level 2 dark magic wizard:

Chaos Lord [597 pts]
(Hand weapon, Full plate armour, Shield, Mark of Nurgle, General, Chaos Dragon, Crimson Armour of Dargan, Favour of the Gods, Headsman's Axe, Poisonous Slime)

Hes gonna have 10 wounds WS7, T6, and a whole lot of attacks and he can get stronger with favour of the gods.
-Headsman's Axe gives killing blow.
-Armour of Dargan prevents multiples wounds
-Mark of Nurgle makes me reroll hit rolls of 6.
-Poisonous Slime gives him poison attacks

The rest of his army is pretty weak with only 3 dragon ogres and an hellcannon as treat with a few small units of monster, cav and infantry filling the rest of the points.

Does anyone have any suggestions how to deal with him?

Also Armor of Dargan is heavy armor but he has full plate by default? Does he get a 4+ or a 5+ armor save?