Getting 3 stars in all events are not required for the achievement, just the regular 100% completion. Stars are earned in every track of every event (those being Races, Battle Race, Takedown, Best Lap, etc.), you get 3 stars for winning in 1st place, 2 stars for finishing in 2nd place and 1 star for finishing in 3rd place, if you get at least third place in every track in every one of these events, you'll get the achievement, combine this with the achievement where you need to finish an event as every character. This can be done in any difficulty, i recommend medium as it gives a decent challenge to the player.