My thinking went like this.....
It would need to be a unit that worked against armour (duhh) but as popping tanks doesn't count towards the Tally, it shouldn't be a
DP, and needn't be a Nurgle unit. I shouldn't consider anything that would compete with any Nurgle units (even Beast of Nurgle!!)-and that basically left me with Seekers, Screamers or Flesh Hounds. Hounds and Seekers are, in a vacuum, better than Screamers, and I thought about Screamers as the choice for this role..
The problem is that seekers are too good!! This might sound odd, but I know for a fact I wouldn't be content to just use them as anti tank units and that would reduce the tally. Screamers, for me, fit the bill perfectly. They are fast, and compared with Hound I lose 1 I and 1 A..(ok, and furious charge..but still). As a simple "throwaway" Tank buster unit, I reckon they'd be great.
I have to say, I didn't consider a Bolt Herald. In my head I had a
HQ of Epidemus,
GUO and Herald of Nurgle. So maybe dumping the
GUO for a couple of Nurgle Heralds, plus a bolt Herald on a Disc might be the way forward..
The only other way I can see of beating armour in a Tallyman list would be to take Kugath for his Necrotic Missiles and Nurgling Infestation..but he's a huge investment; maybe the way forward in a big army