After reading the description, I think that the spells last 16+ combat rounds.
The caster can do multiple attacks. I'd use the skill throw rock to determine if the attack connects.
The target can use dodge to avoid being hit (maybe at half skill)
Since it is a physical attack, armor protects.
You can use the spell to do bolt-like attack (I. e. a number of discrete attacks) or to do continuous attack.
In a continuous attack, the target can then be repulsed (maybe a STR vs magician's POW to resist). In the case of guide fire, there is a possibility for the target to catch fire.