While riding on the baby polar bear. you need to hit an object which causes crash to fly and hit something which can kill him (in my case water).
I did it on "BEAR IT" (second world, the ice portal room) on the third checkpoint, where there is a bunch on nitro laid after it, just after the nitros there is two fallen statues. crash in to the second statue and Crash will fly into the icy water.
i got the achievement then. while trying it again sometimes he doesn't fly far enough sometimes.