Mind Meditation Spell was exactly what Merlin needed to greatly enhance his Mind Power. Mind Power and Spell Models were essential in becoming a greater Spell Caster.
The act of obtaining powerful Mind Power alone but unable to construct a Spell Model was not enough. Laurinka, for instance, achieved the required Mind Power but she was not upgraded to a First-level Spell Caster because she was unable to construct a Spell Model.
On the other hand, it was impossible to become a powerful Spell Caster if one's Mind Power did not reach the required level. Merlin had assistance from the Matrix, hence he did not have to worry about the construction of Spell Models. However, the growth of his Mind Power was not optimal. As a result, Merlin's strength enhancement was negatively affected.