How can I kill assasination targets quickly and quietly?

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?

Throwing knives are best for this. Use throwing knives, then loot thugs to replenish them for free. Also, the best way to use these throwing knives is from the rooftop. Assassinate a target and quickly walk away from view. It is much easier to escape.