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In current implementation you don't need to participate in fight to receive success result. If you wait long enought, enemies will be killed by local patrols and stations and you will only need to visit mission target zones.
This will change in the upcoming version 4.0 as stated by one of the developers here.
Assassination missions
Typically harder than station defence missions since targets tend to be located in positions in zones that are relatively far from stations. Makes boarding a more realistic option though.