Actually, rampages can be easy or insane depending on where you start them. Every rampage has two locations it can be started from, so if you know which ones are where you can find the best location pretty easily.
Always do the ones where you have to blow up vehicles from a place where there's no pedestrians, otherwise you'll have a ton of pedestrians trying to clobber you and making life difficult. Likewise, ones where you have to wipe out gang members should be done from outside that gang's territory if it's possible, otherwise they'll overwhelm you.