Orks v Tyranids sounds fun, though haven't managed to get that one done personally.  A Slasher with 2 Ripper fists and some Stomper friends would probably be a must, along with the Bad Moonz, and a couple of other Shooty clans.  I'd leave the Goffs and KoS at home.  Build monster, impossible to break clans then hang back, shoot lots and don't go the boot until you've smushed 'em for a couple of turns.  Not all that Orky, but the normal rush forward would be bad.
IG follow the same advice, and probably do it easier than Orks.  Tactical Co. screening a Heavy Co will mush bugs, and Hydras should solve nasty Spore problems too.  Have Roughies or Bikes to clean up you back field.  Bio Titans will be a problem, though you too can tool up a nasty Reaver to go toe to toe. ÂÂ

With the Dominatrix and Bio Titans, you decide each turn to go them, or leave them totally alone. If it's GO, then hit them with everything you can. If no, don't waste any shots on them. A Dom with shields up is almost impossible to shoot to death....but she can be swarmed in CC!