The point of the 80% attribute is to increase the HP you’re spending to boost the damage.
Damage basically goes up 1:1 for each HP the skill consumes (before modifiers).
It’s not really possible to hit damage cap without Quickcast OR a ghost-element target.
Getting more HP is how you boost its damage, stacking matk is comparatively costly.
Just learn how to use Bloodsucking with your CC so you won’t get interrupted.
I know it’s not as simple as it sounds, but you have the tools to do it.
Even Earthquake will buy you enough time to top up your HP (unless we’re talking about R8 mobs with tons of mdef, full CON will need more time to suck HP from those).