It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import at the very top of the file.

ZenScript
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import crafttweaker.api.entity.type.projectile.LargeFireball;

LargeFireball extends Fireball. That means all methods available in Fireball are also available in LargeFireball

ZenScript
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new LargeFireball(level as Level, shooter as LivingEntity, direction as Vec3, explosionPower as int) as LargeFireball
new LargeFireball(level, shooter, new Vec3(1, 2, 3), 1);

Parameter	Type	Description
Parameter level	Type Level	Description The level the entity is in.
Parameter shooter	Type LivingEntity	Description The entity that created the fireball, used to get the position.
Parameter direction	Type Vec3	Description The direction that the skull is moving in
Parameter explosionPower	Type int	Description The explosion power of this fireball