About the new Excalibur refine

Hi! I’ve been saving my Divine Dew until today for the new Excalibur refine. Previously I was using Dark Excalibur on my Merric. Now I’ve been testing it with a +Spd -Res Merric and I liked the results.

The problem now is that I was planning to +10 merge him in the future and use him in the Arena, and I hoped Excalibur would be great with the refine. Don’t get me wrong, it is quite good but I’m not going to use a second mage(or a healer) in my main Arena team just because Merric needs it. I assume it’s meant to be used with mages and healers nearby as a reference to his close friendship with Linde(her refine also synergises quite well with Merric)and his romance with Elice(Elice when?), but I expected something that would allow him to take advantage of it alongside Marth as well, since Marth is his best friend after all. I’ve only used melees lately and Excalibur’s effect refine doesn’t work with them. I was thinking about refining it either for Defense or Speed. Do you guys think this would be better if he’s going to be the only mage? I don’t mind spending another 200 Divine Dew, since I’m not interested in any other refines atm. Thanks in advance!