"Stay out of combat" is an objective (usually optional) appearing in various memories in Assassin's Creed IV Black Flag
The best way to avoid combat is to sneak undetected. You can distracts soldiers and guards with the help of Dancers. Also be sure to use stealth and hiding places such as Haycarts. You can also assassinate enemies quietly from Stalking Zones
If the dot on the enemy turns yellow, it means he heard something and will start looking around. Now is the time to hide or get out of sight. If you don't, he sees you for sure and the dot turns red. You can still avoid combat by escaping the area rather than fighting
