I can't figure out how to score the critical hits with the rocket launcher..

You might have to shoot them in the head or fire into a large crowd

Easy. Just go to Hoth, then be with the rebels and hijack the at-st I think or whatever the name is. Then switch to the co-pilot mode. So you'll have the guide able missle and then guide the missile to the at-at(the big one) and hit it 4 times.

Go to Geonosis and shoot rockets at the circle at the front of this battery like thing on the AT-TE

I'm not sure where all the critical hit locations are, but I know how to kill AT-AT's fast. Get a rocket launcher guy, then go to the side of an At-At. Shoot your rocket right on it's neck. That is a critical hit!

Here is a good way to do so.
NOTE: don't capture any CP's unless you want to quit and save, if you want to end quickly, just rush and capture all CP's and wait 20 seconds.
First, go to instant action and pick Geonosis and/or Hoth
Then go to options and chenge the number of AI units on every team to 0
Launch the game
Be droids on Geonosis and rebels on Hoth
On Hoth, spawn at CP 4 as a vanguard (Rocket launcher), get a snowspeeder and land right next to the AT-AT's
Jump out and aim for the "neck" of the AT-AT
That will give you a critical hit, shoot 4 times (or 3 times if you have gotten elite) at the neck and voil�
For Geonosis,be a droid and get to the AT-TE (be an assault droid)
Shoot the AT-TE underside that is in front of the feet on the side (if you can't find it, take your pistol and shoot the AT-TE every where under until you see the critical hit mark.
Then draw out the rocket launcher and shoot the AT-TE in the weak spot.

Hope I helped!!

All you do is take the rocket launcher and press the x button to lock on, what till the gray indicator glows red then shoot. Repeat four times to get the medal. Hoth is the best place to do this.

Hope this is useful

Spider Walker (Geonosis): Where the legs join the cockpit area (the cylindrical bits).
AT-AT (Hoth): Around the base of the neck.
Hailfire Tank (Geonosis): In the machinegun (below the cockpit).
These are all I know at the moment.

EASY...

1. INSTANT ACTION Kassyk (spelling?) (Wookie Planet)
2. Pick Clones
3. Enter God mode cheat (U U U L D D D L U U U L R)
4. Enter Ammo cheat (U D L D D L D D L D D D L R)
5. Pick Heavy Trooper
6. Run up to one of the Amored Tank Droids (Snail looking vehicles)
7. On the very back of the snail in a circle with a line through it, shoot four snails there, because it will kill them in one shot, after four you will get the remote rocket launcher.
8. When it asks you to play as Yoda except but pick Heavy Trooper again and repeat from step 6.