Nobody has declared sunsingers as "to keep enemies at bay". If by that you mean they can zone off with solar grenades then yes they can accomplish that but so can most other classes. Firebolts do too many things too well and the two perks you need shouldn't even be an argument because the other perks are terrible in those skill trees.
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Just like the flame shield which is the only viable option and overpowered because it gives an overshield on hit with great range and even a armor bonus