Since there is no solution for TileMaps seams at the moment (except the one with Integer Letterbox Scaling) I see only 2 viable options (the old ones):
Method 1
Use 9-Patchs, TiledBackgrounds and Sprites to build the Room/Level
Method 2
1. Create your TileMap in another program (example: Tiled)
2. Get an image with the whole level
3. Split that image in smaller images
4. Create 1 Sprite that will have these images as frames
5. Create your Room/Level in C2 on 1 Layout using multiple instances of the sprite with different frames
6. Create another Layer (invisible) that will contain collision Sprites
7. ...
8. Profit
Both methods have advantages and disadvantages compared to a TileMap, but it should get the job done