OK!! Use the Tau ambush tactic thingy. Instead of using Kroot to draw the enemy in, or battle suits, use space marines. There better armor and toughness, they should survive longer and won't be ripped to shreds if they do have to get into CC. The Tau take pulse rifles, Broadsides, Pathfinders and a Hammerhead. Maybe some Crisis to go with the SM and give them fire support, and also some drones, just because they so useful.
SM should take some fast, hard-hitting units (assualt squads in rhinos?) to draw out the enemy form hiding. The Tyranids usually have to come out and getcha anyways, but the Necrons and DE could happily sit out of range of your main attacks, and just shoot.
It would be too risky to do the other tactic, Because the DE and Tyranids would happily assualt you very quickly, and that would be a big mess of Tau body parts.