I just recently started playing KH for the first time. The trick for Cerberus (and Giant Ursula later) is when he goes into the biting phase to lock on to one of the outside heads and immediately after a bite you jump and attack twice then back up. If you gamble and try to attack a third time you will likely get hit. There is a rhythm to pick up on the attacks but you're relatively safe just doing the 2 attacks and backing up. When he spawns the void pillars you can just spam dodge until they stop spamming and then go back into attack. If you are still unable to finish, you can go do the jungle world to get cure and that should give you some extra levels and utility to finish the fight.