large targets do not benefit from cover from other units.
BS shooting should not be a big issue. the carnosaur has pretty high toughness and a bit of armour, and the saurus on top should have enough armour to soak any number of shots. sure, jazzails and volley guns are technically BS shooting and they can hurt alot, but they are probably not too common. (most armies are not skaven/empire, and many skaven do not incluse any jazzails)
the real crux is cannonfire, and other such warmachines that cause multiple wounds with high strength. as mentioned, the engine of the gods is paramount here, and you NEED the charmed shield (or the amulet of izil) for the rider. You might want to include more stegadon to soak fire, or even to use as a shield for cannon balls for the first turn (place the stegadon right in front of the carno, facing sideways. that way he will easily cover him. a cannonball that fails to kill the stegadon will stop dead). But perhaps more importantly you need plenty of warmachine hunters, ie chameleons and/or terradons. if your opponent gets the first turn he will fire at the carnosaur (he would be silly not to) and you pretty much have to rely on luck for him to survive. when it is your turn you charge the cannons with your vanguarded terradons. if you get the first turn, you move in with the chameleons and can hopefully snipe at least one cannon. in either case he should not get to shoot more than once or twice. if you survive that, you can have a field day with the carnosaur. if not... well you are working uphill from there then.
ogres are even trickier. their cannons are rock hard and can be killed by terradons, and skinks will need multiple rounds of shooting to kill it. HOWEVER the ironblaster can only shoot straight forward. not in a 90 degree arc. you could (with luck and skill) box it in with skinks so it cant turn to face the carnosaur (it is allowed RAW and if you want "realism" just see it as the skinks goading and prodding the rhinox, preventing it from going where it should. you may not make any friends doing this though

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If all else fails, just remember that you have to close in FAST with the carno. he MUST charge by turn two if you cant kill the cannons. he will be much safer in close combat.
Lastly, it is worth remembering that only dwarfs, empire and ogres have access to cannons. bolt throwers are dangerous too, but far FAR less so. most battles (depending on your gaming group at least) you wont face any cannons and then the carno is great. if you are up against an empire army with three cannons and two volley guns you will lose your carno. cant win em all
