Userscripts.org

If you don't have this script you must go to player profile, then to click on Spy Missions, then select what spy mission you want to do and after that you can do that mission.
But with this script when you are on the Battle-> Attack pages or on the Overall Rank pages you can spy/infiltrate a player whit a single click.

I didn't added the assassinate option yet, but when a player will send me information's about this I will add.

Good luck using direct spy missions.