Bionicle Heroes

You should easily be able to get the Bohrok and Visorak achievements by simply playing through the game. But the Vahki do not appear as often. So to help, you need to buy
Bonus stage 3. Bonus stage 3 is timed and consists of lots of Vahki. But dont worry, you should easily be able to kill at least 50 Vahki. These stages are timed, located on a very small map, and the enemies will keep respawning. Your objective is to kill them as fast as you can, so you can get the achievement as fast as you can.

To get the 500 Vahki Achievement without spending money on bonus level 3: Go to Zaktan's jungle and play/replay Zaktan's Chamber. Once in the chamber turn left and open up the first room on left. Stay in here shooting big fireballs until you are a hero. When health is full trigger hero support and fire fireballs until you are a hero again. When health is full, trigger hero support. The Vahki will now come out. Become a hero again with fireballs then switch to a rapid fire gun (I used rapid flame gun). Just hold down A button, you don't even have to aim ( I put something on my A button and watched shark week for awhile). You will walk out with 500 hundred Vahki and about 1 million legos in about 45 minutes.

another thing is if you need this and 5million legos you can buy secret level 3 and theres only these guys in it. once you get the hang of it you can get around 40k legos and alot of vahki kills per try.