In Ditas town, Merlin instructed Chablis to prevent anyone from disturbing him.
Merlin began the final step to construct a First-level spell in his room. He simulated the Spell Model with his Mind Power. The Spell Model of the First-level spell, Guardian Monument which was reconstructed by the Matrix was as complicated as Furious Flame back then.
Before this, Merlin's Mind Power had not reached the Second-level. Six Zero-level spells had already used up half of his Mind Power. Thus, when he constructed Furious Flame, the act was quite forced, so he failed in the end.
If he only had four Zero-level spells, he would have probably constructed the First-level spell successfully by now. However, there were pros and cons. If he could become a First-level Spell Caster, his strength would improve significantly as a Six-Elemental Spell Caster and would have a greater potential as well. This was far from what an ordinary Spell Caster could have been.