<p>Hard to say, other than that he's amongst the best. As far as actually using magic goes, he apparently fumbles incantations often, and he's not from the Age of the Gods, so in a straight magic fight he'd probably lose to those with access to AotG magecraft. </p><p>On the other hand, he's more of a support Caster anyway, being able to use illusions, having a version Charisma and apparently being able to make heroes out of others. </p><p>As a side note, qualifying for Grand Caster says nothing about your power, it just means you qualify for the Caster class and possess Clairvoyance (those are the qualifications revealed thus far, others probably exist). </p>

<p>I don't remember FGO saying that Clairvoyance was part of the requirements, IINM Merlin was simply saying that the 3 that we know qualify for Grand Caster all have a powerful Clairvoyance that lets them see into the future or past of their era or beyond. And tthat's not just any rank of Clairvoyance, since a lot of non-Casters have a more basic Clairvoyance and that's defintely not enough even if they also qualify for the Caster class. </p>

He is said to be able to use a reality marble and True Magic