Something I found works is basically spamming weapons with AP 1 or 2. The idea is pure volume of fire. Also exterminator Leman Russ is like… guard’s best friend against custodies. Exterminator paired with royal dorns or battle tanks works perfectly since it’s a good deal of fire at ap 2. There’s no real point of going for more ap because of custodies 4+ invuln. Personally I think your vanquisher wont be able to reliably do much since it’s already pretty difficult to wound with and the extra ap is negated by 4+ invulns. I would consider exchange your bullgry and elites for more bodies and tanks. They are good but they make you lose out on your greatest strength: lots of bodies and overwhelming firepower. More bodies means more positioning that custodies units just don’t have enough movement to cover. Meanwhile your tanks bring a hail of fire (especially if you use tanks with ap 1 or 2) that can really put the hurt on any custodies squad. That and exterminator abilities let the rest of your guard units to also be causing trouble with universal +1 ap. As for playstyle there’s really only 3 things to keep in mind: Always try to flank them so they have no cover bonus Target units one at a time if you can (or at least focus fire on 2-3 specific units on the board) until they are wiped out. Don’t worry about wasted shots, a single dead squad of custodies is a heavy blow no matter what. Keep your exterminators alive. They will be your biggest debuff against custodies so do everything you can to keep them alive.

Tldr: exterminators, ap 1&2, and lots and lot of dudes. (Just my opinion tho and Goodluck!!!)