There are some optional missions where you have to kill some targets (yellow targets) without been discovered.
Theses missions are ridiculously harder for me than others of the same levels, but I've achieved it spending a lot of time hiding from guards (red targets) and after using the hidden blade to kill the target.
The problem is that in block 5, these missions have maximum time and I don't know how to kill the yellow targets faster without been discovered.
Any idea?