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When attempting to pick off Feng Mao as Khaimera, you have to stay very aware of his mana. Feng Mao is one of the trickiest targets for an assassin Khaimera due to his ability to slow you, shield himself, and throw out his Reaping Slash as an escape mechanic. Unless he is drained of mana or your RMB+R will be enough to kill him before he can react, do not waste your time pursuing him as a target.
If Feng Mao does have enough mana to use his Reaping Slash and you are choosing to pursue, pay attention to the direction it is heading. If he is trying to be tricky and shoot it behind you or in a different direction than he is heading, immediately stop following Feng Mao and go in that direction so you can get in range for your Ambush skill to follow up and secure the kill. Unfortunately, if he is able to cast it directly away from you (which is what a competent player would do 99% of the time) chances are you won't be able to catch up and it is best not to give chase.
PS When I clicked to add a tip, it said to give a tip about how to attack Feng Mao as Khaimera. Now, as I am typing, I see the title changed to "Khaimera is strong against Feng Mao." Let me be clear: he is not a particularly good pick or a counter to Feng Mao in any way.