However, just for fun... If I were going to make a list to slaughter Tau, what should I bring?
Tau have 3 main threats and are awesome vs Mech.
Pathfinders. Markerlights are sick. Moreso than any power in the game, Markerlights make marines whimper. One second we are sitting in our Land Raider all smoked up and giggling about how we are about to disembark and eat Tau Crunchies, the next the LR is exploded and we have no cover and are being mowed down by plasma fire. Also, they are hard to kill with shooting. Get them into a ruin and they get a 3+ cover save. Pathfinders need to be dealt with immediately.
Hand in Hand with the Pathfinders comes the Rail Gun. Broadside suit squads and Hammer Heads. Typically you will see a hammer and two squads of 3 suits. The gun is Str 10, AP 1 and can reach any thing on the table. Plus, those pesky suits have a 2+ save and can take lots of drones giving them extra wounds. Tough to kill to say the least.
Mobility. Tau Crisis suits can jump 12", shoot all kinds of hardware, at different targets, then jump away 6". Tau Pirhana can pack melta guns and zoom all over the place. They slice through light armor without even trying and are death to Land Raiders. Plus they have a nasty habit of moving and blocking your lanes of advance.
These three factors lend themselves to Space Marines getting blasted off the table routinely by those pesky Xenos. How do I stop from getting tabled early? Well, Blood Angels have a big advantage over all other marines... Fast Vehicles. If you are going first, move every thing 18" and pop smoke. Sure his pathfinders will take away the cover saves... but not all of them! Odds are that next round you are going to have something in his ranks causing problems. Las cannons are good. Shoot the crap out of the things that are going to block your advance. I know, you are tempted to shoot the Broadsides right? Don't. Odds are you may kill one, but you will have to sink a LOT of shots into them to do it. They are going to shoot no matter what. Move what you can, and use your tanks and devastators to sit back and pick off the pirhana and devilfish. Yes they are probably always going to have cover saves. That is why you need a LOT of shots!
You are taking a crap shoot either way, but take the better gamble. Some of your forces will reach the lines... it is better to stop him from being able to stop you from assaulting than to try to whittle down his guns. You will get to them eventually, and once you do, they go down like five dollar whores.
Whirlwinds are nice too at taking away the pathfinders. I actually may just take one in my army for shits and giggles. It will give me a few points to spend on upgrades, and the right shot can completely take out a pathfinder squad... pretty much eliminating the serious Tau threat.