There's a trick to it. Gotta kill the huge frog mob that spawns after killing the first three bosses. Let Volot "call upon his brood" 5 times (a bunch of little frogs). Some one said there's a "the last of Volots kind have spawned" message, then to only kill half of each wave because if he doesn't have enough adds you fail the event and the frog plushy won't spawn.
