Unkillable bosses?

Arc just has horrible single target dps, especially since you're non-crit. Not much you can do will change that. For end game maps, you should look into having a single target spell on weapon swap or on gem swap (when bosses have separate arenas instead of being random in the map). Storm call or Lightning Tendrils are likely better options for single target. Just gotta get something that works with the links you have.