The Frontier map on the Firecracker gamemode is extremely defender-biased. It's possible to watch both A site and B site from the portal wall area in the lower parts behind A site, and there's a portal wall facing B site on the far side of the map that takes minimal effort to place and replace portals to open a sightline from. This process is even easier if you're playing as Sabrask or Meridian with their respective enemy-detecting equipment.