Hi, that ticket is a duplicate: feedback.arma3.com/view.php?id=14076
but I give you following solution without scripting or strange external workarounds:

1. Create a NATO-unit as a player (your soldier)
2. Create a FIA-unit (your future enemy)
3. Create a CSAT-unit and set "Condition of Presence" to "false" (the unit is not physical present in the mission)
4. group the FIA-unit with the CSAT-unit (not the other way around) via the group connector (press F2) -> Now the FIA-unit becomes a CSAT-unit (and now is your enemy)
5. preview the mission and you will see, that the FIA-unit is attacking you.

If you want to fight against a created FIA-group (inserted via F2), the single physical not presence CSAT-unit has to be set from Private to Sergeant or higher, because the group-leaders are always Sergeants. It won't work, if the rank of the CSAT-unit is lower than the rank of the FIA-group-leader.

If you want set waypoints for the now enemy FIA-unit(s), you have to set them for the physical not presence CSAT-unit.

Just have a try, it works. With that solution, you can fight against your own troops without causing friendly fire. They will attack you, if they see you.

If it won't work, I can upload a working little mission file in here.

Best regards.

I guess it couldn't hurt