To make the boulders roll faster, place a brain on top and only slightly towards the direction you want it to go. The brain will slide towards that side, propelling the boulder. OR, chances are, you only have to put a brain on the side opposite of a zombie, for it will propel the boulder even faster.
A helpful strategy (especially in the later levels) is to scroll down occasionally. When you return to the actual game's screen, the spawned units will typically be bunched up due to lag.
 "Penguin Overlords"){kind=icon}
 "Flight"){kind=icon}
 "All We need is Brain"){kind=icon}
 "Bridge Tactics"){kind=icon}
 "Multitask"){kind=icon}