I found a few quick ways to get some of the Achievements.
Yee Haw - Ride all mounts and vehicles.
Once you have 10 Gold Bricks, go to the Bonus room and build all the doors that are available to you. One of them will lead to Lego City. Almost all the vehicles and creatures in the game can be found and/or built here. The only ones you have to find in-game are the floating Battle Droid platform (which can be built near the end of the very first level of Phantom Menace) and the little hover buggy things (there's one at the start of the Darth Maul level).
About halfway through the Kamino level of Attack of the Clones, you'll ride up an elevator into a room with two hanging chairs you can Force-rotate up into the ceiling. Just past this room you'll be faced with wave upon wave of flying droids that shoot like crazy. As long as you don't use an R2 unit to close the pipes that are letting them in, you get an infinite supply of these things. Just plug away with the X button with a lightsaber character, and then with a blaster character. It takes a while, but because of their rate of fire you soon clock up the kills/dodges.