if the guard's big nasty tanks are giving you trouble, the kit-out a carnifex with a twin-linked venom cannon (you would by 2 venom cannons from the list of weapon symbiotes) don't forget your enchanced senses either!
this gives you the most accurate long-ranged tank-hunter in the 'nid list. generally speaking, every second shot that hits a russ will glance. as your cannon is twin-linked, you can usually back on at least 1 glancing hit/turn! you don't have to destroy the tank, just stop it from shooting!
the other point about 'fexes vs a guard army; keep them cheap!!! the guard's main strength is the sheer amount of big guns they can bring with them... guard are excellent at killing your monstrous death gribblies! by keeping your up-grades/biomorphs down to a minimum, you're denying your opponent one of his biggest advantages - namely that his big guns can pay for themselves by killing a single big bug! a basic carnifex is just as hard to bring down as the fully tool-out, 'uber expensive one... (and if you can keep all your 'fexes under 140pts, you'll have that many more pts to spend on even more bugs!!!)
in fact, against guard don't even bother with an assaulty 'fex cause he'll never get to the guard lines! your opponent isn't going to come out and meet you like marines will... just tool-up your 'fexes as large, solid gun gribblies!
a flying hive tyrant w/2x twin-linked devourers, toxin sacs and enhanced senses will reap entire squads each turn! (for those keeping track, that's 12 S5 shots, re-rolling to-hit and re-rolling failed wound rolls!!!) throw on psychic scream to help negate a number of the guard's Ld-boosting abilities such as a nearby officer or close order drill formation!
in general living ammo is your friend! guard will easily out-gun you... however, taking lots of termagaunts and warriors w/devourers which both have the 'living ammo' special rule will really help to ensure that each shot you fire will hurt those squishy guardsmen!
i'd honestly avoid a broodlord vs guard. unless you invest into some flying or leaping warriors and gargoyles, your broodlord will be unsupported and will easily be swamped by the more numerous guardsmen... gargoyle squads are acctually quite good vs guard as they can have solid numbers and they're lightning quick! (as fast as hormagaunts
) bio-plasma is a plus and can be put to good use to pick off any I4 officers carrying power toys!!!
warrior squads offer more wounds so it will take alot more to remove your precious synapse!
genestealers with scuttlers is a guardsman's worst nightmare! the bonus scout move, plus fleet each turn can help these guys close with any armoured targets very quickly! don't bother with the expensive extended carapace either as the guard can again bring a horrific number of heavy bolters to bear!
keep your broods to roughly 16-18 models each. this ensure lots of bodies to soak-up incoming fire, but is a bit more manoverable meaning you can take much better advantage of any cover! more broods also means more seperate targets and scoring units!
hope this helps!
cheers!