Kimeramon is a Raid Boss located in the Kaiser's Laboratory. You will find this instance dungeon in the Western Area's Dark Tower Wastelands. In order to enter it, you have to find a "Kaiser's Laboratory Warp Gate", this item can be taken from any monster on the map (be aware of the drop cap). After you got one, talk to the Digimon Kaiser near Kurata (approximately in the center of the map) and choose either difficulty you want to play the dungeon.
There you'll meet Kimeramon... and some of its friends (summonned from its bodies parts).