You’re right, while I’m still putting together the build (still at the stage of a partial pre-final Champion of the Light set), using Grenado with rata, it certainly would be helpful to have 2-3 additional rata skills to use on bosses (while Grenado is on cooldown).
But as Gnomish_inquisition pointed out, this build implies that you can very much stack blackwater cocktail to kill the super bosses:
Retal Commando - BWC caster | 20s dummy | 7k hp/s - Grim Dawn / Classes, Skills and Builds - Crate Entertainment Forum