Attention 40K Players: I need some assistance
 

I'm 90% certain monoliths cannot deepstrike.

Okay, go to your buddy with the Space Marines. Tell him to follow these steps.

1. Give his marines the "Trust your Battle Brothers" trait. Toss away psykers or something. Give all sergeants power fists.

2. His entire fast attack category? Fill it with assault marines.

3. Keep a very large number of marines available for drop pod assault.

4. Kit the command squad out for close combat.

5. Take two devestator squads, each with two heavy bolters and two lascannons. The heavy bolters can tear through his fire warriors. The lascannon can demolish his broadsides.

6. Elites: Close Combat Terminators. Deep Strike them into the heart of his forces.

7. If he took no broadsides, put all the tactical marines and the command squad in rhinos. If he took broadsides, you're probably safer risking it on foot.


Okay, now to your tyranid friend.

His army is simple.

Gaunts. Take nothing but gaunts. Gaunts and Hive Tyrants. His tau can shoot, but eventually something will break through.

Now for you. Take a Destroyer Lord and a normal Lord, each with this revival orbs. Put the normal lord with a squad of immortals, and the destroyer lord with a squad of destroyers. Max out on warriors and immortals.

You stand and shoot, m'kay?

The tyranid friend rushes like mad, with you and the space marine shooting away like hell. He's forced to choose to kill the onrushing hordes, or to deal with the guys that are already gunning him down. The necrons will get back up, and the Marines shouldn't fall at all. Once the tyranids have begun to engage the doubtless large numbers of troops, the space marines are to kill the Heavy Support, then the Leaders. The necrons just stand and pour on the fire power. Essentially, the 'nids take away the meat-shields, the space marines kill the important stuff, and the necrons ensure that no matter what he's doing, he's facing damage.

Okay, at it's most basic, this is the strategy for playing against Tau.

You can't out-shoot.

You definitely can out-hit them.

Get in close and hit them to death.

This is like another language to me. All these weird words and such. Interesting, though. Your enthusiasm almost makes me want to learn this game.

I'm a predominate tyranid player, so against Tau? Genestealers and Hormigaunts.

Tau will outshoot you any day of the week, so don't bother with any of that jazz. Get yourself a couple Carnifex/Tyrants with Venom Cannons or Barbed Stranglers to stop those Devilfishes from moving, and make sure you get the giynormic floods of Hormigaunts to go first. Their 3 inch close-combat range makes a world of difference.

And finally, Genestealers. Genestealers Genestealers Genestealers! Three attacks a unit, Rending, fleet of foot? Shit, if you know you're going to fight tau just don't bother with the Extended Carapace upgrade and just get more and more Genestealers. I like adding Scything Talons for the +1 attack, but that's a preference.

EDIT: Also, isn't this a bit more..."General"-ish?

Plague Marines.
Backed by plague terminators.
And maybe some zoats if you have them (yay zoats!)
That is all.

What? No orks? Wheres the ork love, mate?

MAKE IT ORKY!

Well, if you really want to obliberate him, go the dark, cheese-covered path of power-gaming.
Necs:
2 deepstriking monoliths (yes, they can, but count as moved!), using their particle whips (which they can use, even after moving) to snuff out his heavy vehicles and if there are enough models in 12" radius blast them with your gaussflayers (or how they are called, those d6 hit thingies).
2 Lords with regenerators should keep your Infantry save.
If you feel really mean, include a veil of shadows to teleport an Immortal-squad to wherever you need it (with high S, ap4 and assault they should be able to fend off any attempt of counterattack by those nasty fire warriors).
Deepstriking flayed ones will break his formations and cause some havoc by also breaking serveral squads.
If one of your squads should be annihilated with no regenerator around, just ´port them to your monolith, to a) get an "Ibb" roll and b) get a reroll if needed.

Tyranids:
I just say: Godzilla.
Yeah, really. Those damn "only-big-ones-that-can´t-be-insta-killed"-lists are hard to beat. Most Tau armies have a big problem taking out masses of good armored troops (3+ or better).
Don´t forget Synapse (sp?) creatures make your guys immune to insta-kills!

Space Marines:
I assume he´s playing UMs, right?
Ok, here we need the good old droppod torrent (drop everything that can be dropped, deepstrike the rest) as described by DarthSS.
Include an inquisitor! Yes, inquisitor. One of the Ordo Haereticus (witchhunter), with minimum equipment, because his only purpose 1 special item:
Liber haereticus (Ld-tetst to auto-chose side).

Those aren´t the fairest ways to fight, but if he really deserves it, as you said, that´s ok. :D