FASA Games, Inc.

1) do i have to achieve a good success versus projectiles when using shield mist?

2) willforce is only have to be used once: when casting the spell, right? (not every time when using it to deflect blows)

1) Shield Mist shares all aspects of the Avoid Blow talent or skill replacing it with an Effect Test without the strain requirement. So yes, it requires a Good Result to avoid missiles as per the talent and skill.

2) Willforce is used once during casting. See Effect p.141. The example uses Doom Missile. If he decides to use Doom Missile with Willforce, he takes 1 strain, then the spell's Effect Test is raised by the magician's Willforce Rank for its duration.