... okay, I still don't see it. Copying Medea's stats for comparison--as the pre-eminent magic-focused squishy Servant--we have:

Strength: E	 
Mana: A+
Endurance: D	 
Luck: B
Agility: C	 
N. Phantasm: C

So... you're saying he somehow has more mana (naturally, ignoring her preparations) than even her, but is somehow weaker than someone Rin can beat up? 

They're... too weak. The elements are sort of useful, but Caster is more or less the weakest class anyway, and even Medea's ability to spam Servant-vaporising lasers and about a dozen other things didn't help out that much. Their divination skill is useless as it changes the future (and divination in an RP doesn't work anyway because knowing what's going to happen is hard), and their NP doesn't actually do anything worth noting, let alone being worth an A-Rank NP.

Please buff them. XD