Ive been wanting to Destroy some fnix bases, but i dont know how to start a base assault. Does anyone know how to start one?
Just attack the base.
If you get close enough, the base assault mission starts automatically.
Be prepared. The mission aborts, if you leave the area, even if you die and respawn at a safehouse. If the base wasn’t at full level since then, it probably could get an upgrade if the mission is aborted.