Digimon World: Dawn and Dusk/Explore Limit Valley
Exploring Limit Valley! This area is a great area to get closer to Champion.
When you enter, go up into the next area. ...Second thought, this area shouldn't be too hard to navigate through. Dead ends are common, but this area has no key or puzzle.
You can encounter (on your first visit) Koromon, Agumon, Biyomon, and Renamon on the bright side. The dark side has level 20-25 Digimon, so be cautious in battle. This side has Seasarmon, SandYanmamon, another version of Goburimon (witch is a technical type, so be wary in battle against it), and occasionally Renamon.
At the end (save!), you'll see mystic energy. Fight his level 28 Seasarmon (300 exp), and he'll make you fight 3 SandYanmamon! But before that happens, your friends pop up and take them on, while you fight the boss of this dungeon... Grimmon!!!
Grimmon is level... 40? And is suprisingly a Champion. He's unobtainable. Grimmon covers 3 zones, so if your Gaomon has Small Tornado, or if your Biyomon has Suprise Blow, you'll be able to attack him twice, but the zones each have different weaknesses, those of which CHANGE when he uses AT change.
Winning gets you 300 of 4 different exps, bringing you to a whopping 1200 exp total! Now Lunamon should be able to digivolve if you're playing Dusk.