Well first off, they would not be top tier armies if they were easy to beat. The top armies are tough as nails, and pack a punch like a truck, so let's try to see if there are ways to beat them. Sometimes you are not able to stop them, but you can slow them down a little bit and hope your strategy and tactics will help carry the day.
#1. Fight fire with fire
You can always beat top tier armies by bringing you own top tier army and this is what most people do. They are not creative and just follow the tends and fads of the latest and greatest armies. I would never do this myself since I am an originator, not an imitator but it works for a lot of people. Be warned though, there are several bad consequences to doing this. One is that you will go to tournaments and all you will see are these types of armies and that is all you will play against. Tournaments are already heading in that direction with it being just a sea of Mech Guard, Space Wolves and Blood Angels. Boring! The other problem is that these armies are alpha strike armies (Have an awesome amount of firepower to cripple their opponents on the first turn) and who wins or who loses often goes to whoever wins the dice roll to go first.
#2. Build a list to counter the top tier armies.
This is very hard, but it can be done. Here are some things you need to counter those armies
#1. There is a ton of missile launchers floating around so you have to make yourself immune to them. This is not as hard as you think. There are several units that Missile Launchers have trouble killing so let's go ever them:
AV 14=Land Raiders, Monoliths, Battle Wagons; Lascannons have a better shot at taking these vehicle down, but AV14 is almost immune to strength 8 shooting. That is why Ork Battle Wagon builds are some of the most common these days because they are the only thing that gets them to where they need to go. If you look at some of the winning lists at tournaments you will see that they have very few lascannons in them and those are in fragile razorbacks which you should be able to take out in a gunnery duel.
2+ Save=Oblits, Terminators, Broadsides: Any of X-wing armies can stand up to missile fire. Loganwing, Deathwing, Vulcanwing what have you. Terminators can take a ton of fire and still be ok. Oblits and Broadsides can too, but when they do roll a "1" they will get insta-killed which is not too great, but with that 2+ save they are still quite durable and can stand up to missile launchers. On average it takes about 11 missile shots to kill 1 oblit.
#3. Reach out and touch someone.
As they say, it is better to do unto others before the do unto you!
With assault armies there is a bunch of nastyness that needs to be taken out on the other side of the board and you need to get to it in a hurry and destroy it. So if you are an assault army you need to get to where you are going to in a hurry because you will not have any time to be standing around. That means that you either need to be super fast (Dark Eldar) or have some deployment rules that get you to where you need to be safely like Deep Strike, Outflank, Infiltrate etc.
For shooting armies you need to take down their threats at range. I myself prefer to pack a lot of Lascannons in my armies. Sure the kids these days love their melta, but in a lot of cases, it is not only hard to get that close, but you don't want to be that close. If you are able to melta a Battle Wagon you are too close. They should come with a bumper sticker that says if you are close enough to get an extra penetration dice, you are too close. Enough Lascannons can stop a battle wagon rush before it even gets started. As far as stopping the soft targets you will need something that can take them down as well because you will need to kill infantry at range. You will have to deal with long fangs and lootas and a bunch of other units so it might be nice to have answer to the question of what am I going to do about them? Since they will be in cover most of the time it is best to deal with them with a fast moving units that can flame, come in from different board edges, or throw a lot of lead down-range.
#4. Go against the Meta
Everyone knows that the best way to win is to Mech up. There are so many vehicles out there these days that armies are filled with nothing but tanks and Melta weaponry. So you need to go in the other direction and leave the tanks at home, and take an army without much in the way of high priority targets were Melta can hurt you. This is why my foot eldar works so well. Without wave serpents to shoot down, there is not too much in the way of high priority targets to shoot at. I have a lot of junky troops and the rest is durable wraithlords and an avatar. You can also do it with the tyranids by building a little bug horde, or a lot of Ork Boys. Since there has been a huge meta shift to mech and anti-mech weapons, if you eschew them for a horde army, you might be in good shape. I played against a Tryanid player at the So Cal Smackdown last month and when I hit him with my Warlock's Destructor he remarked that no one in his other 4 games that he played brought flamers.