To do this, you need nine HP disks "B", and nine MP disks "B". Also, your digimon (Dorumon, Agumon, Veemon, or Guilmon) needs to be at level twenty-one. Now that the level and disk message is out of the way, let's get down to business. In the terminal area, go right. Then head upwards. Stand in the middle, then go down and left (just a little bit). Talk to the digi-elf and say yes. Go to the outside world gate. You should see it changed. Go to the Battle Arena. Hint:(defeat all the digimon in the arena for EXP points). See where you can jump onto the ledge, go there. Enter the Barbarian Cave. (I should've said this earlier, but you also need nine gate disks.) You should see a clock at the top left hand corner. Beware: there are powerful Numemon in there. If you're trapped, defeat all the Numemon to proceed. Hint:(If you're low on health, use a gate disk). Also get the maps (if there is any, because I can't remember).When you make it, beware he's tougher than before (if you fought him before, he was easier). I'll explain his attacks after this sentence. When you defeat him, if he dropped a key (I can't remember), pick it up, because you'll need it.
Mammothmon's attacks:(X indicates when its time for you to attack.)
1. His breath will hurt you, so watch out, because it can hit you multiple times.
2. He'll charge at you, it's easy to dodge also, (because it's a slow attack). X
3. He'll swing his trunk like a maniac, so you don't get much time to attack.
PS. You can probably hit him a maximum of two times with one shot.