As you start with 150 sun, in some way, this is quite easy. If placed correctly, the first Wall-nut may counter any first zombies spawned. It may waste a Wall-nut but will make it easier later in the level. When Twin Sunflower is unlocked, placing a Wall-nut and then a Twin Sunflower should be easy as your sun will be quickly replenished, thereby earning Safety First easily.
Trivia
Safety First, Traditionalist, and Scouter were all about placing a specific plant first.