In general, most damage dealt at any given moment means most x% of scaling dealt in the lowest ammount of time possible.
Also, when compressing your skill rotation "even further beyond" by the means of cooldown reduction you also have to appreciate if the stats discarded for it all are worth discarding. By "it all" I mean the whole pool of benefits gained via the newly introduced stat in relation to what you could have been able to acomplish by the means of a different stat. Therefore it all depends on what the point of interest is/what you're looking to gain that would facilitate "better winning": is it better room clear? is it more burst? is it more sustain damage? is it better sustainability? In the current state of Elsword, for example, being the best DPS possible really doesn't weight as much as it did in the past because nowadays most of the winning is done through room clear. It may also depend on what "better winning" means for you, personally: does it mean more chill runs? does it mean faster runs? or are you looking for something else, maybe?
As a general rule of thumb the only skill in IN's kit that can be considered a bossing skill would be the unmoded Shadow Bolt, everything else they can do is just support to it and it all, execution time considered, does nearly the same amount of damage output per sec.
Purely DPS speaking, the thing about FI2 is that as long as you don't finish your kill by the end of it you will gradually start bleeding damage output because the skill rotation would not be as compressed, because this skill takes a long time to finish. On the bright side, even if you don't find yourself in a scenario where you can make use of the burst part of its potential, it still makes for a good defensive skill, for defensive gameplay, for it has really good MP gain. That being said I almost never use this skill consistently cuz on the minus side: the long execution time renders it a bad room clear abilty (unless you're very weak and fast room clear isn't even a thing for you in the first place); as said previously, if you don't finish up your kill by the end of it you end up loosing DPS; it disrupts Shadow Bolt usage by tossing movable mobs away.
IN is one of those classes where skills have to harmoniously flow into one another cuz otherwise none of them are good standalone skills (except Shadow Bolt). Which further implies that there are no bad skills and good skills, only skills that make for a good final product or not.
In terms of CP skills, at least from my point of view, this is how the cookie crumbles: