Giant Koromon Questline.
1. He will appears in Village of Beginning after you recruited koromon (normal one)
2. He is in Boot Jungle
3. He is in Oasis
4. He is in Jagged Rocks
5. Now he is in Dina Plains. He’ll evolve into Agumon (Black) after talking to him and then we need to win the fight with him (Lv. 15)
6. After that, BlackAgumon’s House will appear on the map, go visiting and talk to him.
7. Back to Boot Jungle, fight him again (Lv. 25)
8. Go to Sizzling Desert, fight him again (Lv. 30), he will be sad after all his losses.
9. Visit Digimon Village and listen to some villagers’ concerns.
10. Go to Sewers, now he evolved to Greymon (Blue), fight against him again (Lv. 40) ** He has ultimate stage stats **
11. Visit Digimon Village again. Talk to all concerned Digimon then Piyomon will tell you to go to Mecha Plants.
12. Going to mecha plant isn't actually necessary. Skip to step 13.
13. Go back to the village and talk to Hina. You now can access Server City.
14. Go there, our good friend now evolved to Metalgreymon (Blue) and rampaging the city, we need to stop him. (Lv. 50 (?) + Mega stage stats)
If you have trouble following this, try grindng for more Digimon Defeated number first. Around 500 should be fine.