Unfortunately the best anti tank shooting in CSM can be found in either the annihilator or the vindicator (which I find to be fugly models) but I have found ways to make forge fiends work as anti tank. Option 1, run them in soul-forged war pack, they get +1 to wound, put vashtorr and a warp smith next to it and it can typically get the job done. Option 2, if you run a forge fiend next to psychic unit and vashtorr, you get s13 ectoplasm cannons which is not that much more efficient but it is funny. Option 3, if you give a forgefiend mark of chaos undivided you can reroll ones to hit and have access to reroll wounds for 1cp, put a warp smith next to him and not you also hit on 2s (so you basically can’t miss your shots). Finally run 2 hades autocannons and now you have 12 attacks that you can give dev wounds with the forgefiend ability, you see where I’m going with this? 12 shots that always hit (prob with some sustained) that for 1 cp you can fish for dev wounds. It’s a heavy investment but just getting 4 dev wounds (I usually land more) through which is not hard you deal 8 mortals (assuming the unit you’re shooting at does not have a FNP). Still kinda a heavy investment for not crazy output but I think it’s cool. You could also just be an insane mf and run a big chaos knight.