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I have a friend who plays guard, and his favourite strategy to counter my TWC deathstar is to get a blob of 20+ guardsmen and have them fire at me. If I assault, they just bog me down. Now, I need some solutions: I have an Imperial Knight, and its Rapid Fire Battle Cannon is great for blobs, but I tend to use that on his tanks, so I was thinking: Is a Whirlwind a good investment for blobs?

How do the TWC just not murder everyone?

And yes, Whirlwinds are good vs guardsmen.

How could you be having issues with that?

An Imperial Knight is a hard counter mostly because of Stomp, so that isn't a bad option.

In my experience 20 man blobs disintegrate too fast dying on the turn they were charged. 30 man might live to hold you and die in your turn. What you should be worried is a boatload of 10 man squads all over the place whose sole raison d' etre is to block your advance and die in your Assault phase leaving open to be shot by his tanks.
On your question however a battle cannon is crap against tanks and your Knight needs to get stuck in melee.. If you add a Knight to the threat of your TWC his blobs will collapse trying to cover all approaches.

I'm surprised that A: you arent murdering 20 men whole sale, and B: that they arent immediately falling back. One of the best ways to kill guard is with ledaership, unless they have priests. If you see a 20 man group coming your way just do anything to cause them to take 25% so they break (and they likely will).

I used to always tarpit TWC with 50 man conscript blobs and a priest against an opponent who LOVED to assault them for some unknown reason. Once you are in theres not much you can do to stop a priest backed blob beyond throwing more units into it to hasten its demise.

As stated before , Battlecannons are terrible anti tank, Long Fangs with Lascannons are your friend (plus that nifty ignores cover helm)

I thought thunderwolves just mulch things, anything in there path?

How many thunderwolves are you using in a squad? They should chew through 20 guardsmen like nothing.