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What are good tactics against armies such as orks, IG, dark eldar?
Green Tide will consume You all!
Barbed Stranglers are good against those other swarm armies. Same with Biovores and spore mines. If you have number superiorority then you can probly win.
Against IG, if they dont have a lot of tanks. You can exeptional size your Tyrant. Its expensive but at least you dont have to worry about getting killed by lasgun fire. 40 Lasguns are going to do something to your tyrant, but not if they cant wound it. Plus the heavy bolters will be wounding on a 6 and such.
Sifer already said the basic tactics. Against swarm armies you dont want a lot of big things. You have to swarm them back. Warriors are good because theres a lot of them. I would strongly recomend getting some with ST and RC. Biovores are very nice. Use the Poisen and Frag mines. Remember even if you dont hit those mines still stay on the board. So they can fly into another unit or cause some chaos.
Orks, there is a lot of them. They cant hit the borad side of a barn, but in numbers they are just as acurate as Space Marines. 30 Orks fire, 10 hit. 15 SM fire, 10 hit(this is at about same point value). Raveners are cool to use. Although I dont have them yet I would figure that they will cause major damage to Ork army. Lets say in a squad of 5, 3 make it into combat. You attack first with 15 attacks, assuming that you have ST and RC. You hit on a 3+ wound on a 4+. They get there 5+ or 6+ armor save. Plus remember any 6's you get is an auto kill. A Carnifex with a Barbed Strangler is good against orks. Although you have to hit first. But if you do you can take about 1/2 to 3/4 of 20+ squad.
against orks you will have to watch out for the waagh-test-passed-close-combat-initiative8-monster warboss as this guy will give quite a challenge even to your hive tyrant not to mention their nob bodyguard. add mega armour into that and it gewts worst (dont say they are crap because they are slow, slow movement doesnt matter in assult V assult and besides, orks CAN outshoot tyranids by a mile)
Orks can outshoot Tyranids? You're mental mate! Tooled up nids can be shooting Strength 3 Ap5 guns with BS4, whereas Orks get the same range, same sorta guns, and BS..2. And equal(ish) numbers. One squad of VC warriors can clear out most Ork vehicles and with it their only half-decent shooting weapons in the first couple of turns.
As for speed, it does matter in Assault Vs Assault, for reasons such as who gets the charge bonus, and who gets to fire their measly weapons at the enemy first. And the more agile army can choose to place the assaults where they want them. However, if you're not using the TVR, then the Warboss can be bundled into a truck and shot forwards 24" and then get out and assault the enemy. And points for points, it loses to the Hive Tyrant. Orks can spend many hundreds of points on the Warboss and Retinue, but if it doesn't get the charge then the Tyrant can pick it out and kill it before it can blink. (Not under TAR though, AFAIK). And the Tyrant Guard are awesome as well, especially if you give them the Implant attack for +6 pts.
Orks really struggle at taking down big nasty things, so a T7 mutant would be able to laugh off most Ork attacks if possible. Otherwise stick a normal Winged Hive Tyrant into close combat with their largest units and wait to charge everyone else in and finish the job when they eventually roll four 6s.
Don't take Genestealers, go instead for cheap gaunts with Scything Talons and if points allow, Bio-plasma. A gaunt that gets 4 attacks on the charge will irritate the Orks a lot.