Games

Building nurgle armies is easy. You need to ask yourself two question:

Is the unit riding a rot beast or a dragon? If not, is it wielding a huge two handed AP weapon? The answer to one of these must be yes, otherwise its a trap option. The only exception is chaos sorcerers.

Now the game tries to trick you. It offers you units that wield huge two handed weapons in one hand, like trolls, ogres, chariot riders and great unclean ones, that's a trick and you can't fall for it.
It may try to convince you that shields are a thing you care about, but shields are for people without 5000 armor, a squintillion sources of healing and rot beasts.
Demons are always the subpar option if there's a mortal option.
You can substitute one unit for something like a beast of nurgle if you really must. But nothing fancy. I better not see you try using something like rot flies.

Your only threat is heavy AP ranged units that is faster than your infantry, and the answer to that is rot beast (Either in the form of rot knights or a hero/lord on one) Rot beasts are also your answer to getting flanked.

You CAN use normal chaos knights if you don't have rot beasts, but marauders on horseback is a trap.

Skaven have two things that do anything but mildly inconvenience you. Weapon teams and the spell Warp Lightning (No other spell matters, if it was ikit claw you also dont want to be nuked but this is moulder). Therefore your first priorities are sending cavalry and heroes/lords at those two things. Once they're gone there is literally nothing skaven can do to you. You can tarpit five full stacks of stormvermin with some of your infantry without trouble, if their rat ogres charge you they die.