Digimon World Dusk: Union Quest: Explore Limit Valley.

After you have done at least two species quests, this Union Quest will become available to you. Talk to Piedmon and accept the quest. You cannot cancel a Union Quest halfway! Make sure you get your Digimon(s) and items/equipment fully ready.

Once you ready, go to Union Room to meet Chief Julia. I recommend you to digivolve your Digimon(s) once they reach their maximum level or achieve certain conditions for their digivolutions.

Chief Julia will ask you to explore a new DigiArea called Limit Valley. So, hit there and go right to open a box contains Digiar150D. Ignore the ladder and hit northwest. There is a box with a Digiar150S. Take it and climb up the ladder, then hit northeast twice. Take Digiar150T on your right and continue the path to climb up the ladder and move to southwest.

The next dark area filled with tougher Digimon(s), so be careful. Climb down the ladder to get Broad Shield. Climb back up and move to the left and climb up the ladder. Save your game and approach the Mystic Energy. It will summon a Seasarmon.

Seasarmon
HP: 1100~
EXP: 330 Beast
bit: 100 bits
Item: Digiar150T
If you have two Champion-Level Digimon(s) then it will a piece of cake. But be careful with its Flash Ray, it might put your Digimon into Confuse.

Then, the Mystic Energy will summon three SandYanmamon, but don't worry because your friends will take care of them. Unfortunately, you will be facing with the Mystic Energy that turned into Grimmon.

Grimmon 
HP: 2500~
EXP: 1200 EXP (each 300 EXP for Dark, Dragon, Bird and Insect respectively)
Bit: 1000 bits
Item: Digiar600S
It takes up three zones, so if you have an attack that can hit 3-zone, use it! My Togemon helped so much with her Needle Spray attack, together with Grizzmon's Gatling Punch.

Later, Grimmon will leave and say that it's only just a beginning. Take the green portal and back to the city. Go to the Union Room, then to the Dark Center for 110 Tamer Points, 7180 bits and Love DigiEgg.