L-group on the 4th line
The black group resembles the L-group but it is on the 4th line instead of the 3rd. It also resembles the Carpenter's square for which it is missing a stone at a. The carpenter's square being ko and the L-group being dead, it is no surprise that this group can be killed, while it is not easy at all to find the sequence. Most hasty attempts will end in ko as well.
Realistic situations would probably involve a cut instead of a firm connection, i.e. the stone being black. Also the stone is important for containing Black.
This analysis is based on the solution by An Younggil in gogameguru's problem 107: http://gogameguru.com/weekly-go-problems-week-107/.
It is carried out however by applying How to approach a life and death problem. (Panda Sensei analysis was later added as 3.)
Panda Sensei shows a different, simpler way to kill.