these are just basic tactics that are for newbies that dont know how to kill who.
Eldar-Whirlwinds and Librarians. these 2 are very effective at taking down the eldar aspect warriors and their phycic powers, especally now that they can be used against warlock powers too.
Vechicle IG: Predators, simply because errr....they're predators and an assult squad with meltabombs.
Infantry IG: Normal marines and a wirlwind i find work well.
Tyrinds: once again the ever handy wirlwind, maybe a predator to take out the big bugs.
Nercrons: Take as much plasma weaponay as you can, take out tomb spiders early.
Orks: Whirlwinds, land raider crusader
Dark Eldar: stay in a solid formation so they cant smash u 1 at a time. Heavy weapons to take out raiders then a wirlwind to wipe out they're infasntry.
Tau: rush forward sticking to cover if availabe, deep strike anything that's alloud. Terminators with heavy flamers deepstriking down to the Etherals unit and firing is very painful to the Tau.
I havent mentioned Sisters of battle, Chaos or other Sm's because i dont actrully know any1 who plays SOB and SM's and Chaos have to versitile army lists to know what you'll need, but if u know that u'll be playing against a certain chapter then do what their weakness is ( all the Chapters with their own list or codex have a weakness)
And thats my 2 cents, also this is just what i have found works and i clearly like Whirlwinds alot so feel free to just add to this list .