SMITE

As Sun Wukong, use your Ultimate and create a decoy. The decoy will fight for you while you're invisible and healing (a side effect of your Ultimate).
Your decoy has to kill an enemy for the Achievement.
As far as I can tell (I'm not too familiar with Sun) the decoy auto-targets Gods and ignores minions so this might be better to boost as the decoy can die pretty quickly.
If you can't boost try to sneak in while your teammates are attacking the other Gods and hope to steal the kill.

As Sun Wukong you need to use your ultimate ().

This will teleport you to a cloud to heal, but will spawn a decoy. To unlock the achievement you must kill an enemy with your decoy. You can't control the decoy so you need to utilise it on someone with low health.

Do not use your final strike to kill the enemy fire you want the achievement.

Can be seen perfectly in the video.

This is easier done mid game in arena, start off by building warrior tabi then right into frostbound hammer. while an enemy is low ult then your decoy will chase down the kill. if you dont have it by now finish your build of by getting daethbringer, bulwark of hope, void shield and mystical mail.

side note - your decoy is instantly killed by hard cc eg. stun, mesmerise ect.