Just cannot manage to do this one! Any tips?

SPOILER:

first row: C 6-8, 18-20
bottom row: C 4-7, 12-15, 19-21.

Thank you, Gudi, for the hints.

Mostly logical. Two possible solutions at the very end. Thank you for the puzzle!

The spoiler gives you everything but the head, which has 3 possibilities (pick the symmetrical one)