Slim in his current stage is just broken. One aspect of it is his inconsistent healing. Against Behemoth it’s godlike, against Wraith, Gorgon and Kraken not so much. Obviously this is due to the spread of the shotgun which is very high at maximum firerate.
Instead of simply decreasing the firerate (by a lot), how about just having the center pellet of each shot absorb life energy and the other 7 do nothing but damage?
As you most likely have noticed, all the shotguns have 1 pellet that is pinpoint accurate and goes exactly where you aim. If that would be the only pellet that reduces the healburst cooldown, you can fire as fast as you want at a very small target. If you have good accuracy, you have the same healing output as if you fire at a big target.
So the pacing of your shots would affect just damage, not healing. More healing, less damage. More damage, less healing.
Obviously, that one pellet therefore should regenerate much more (about 4-5 times what a current pellet does).