I assume you have lightning spear? That spell carried me throughout my entire playthrough.
I tried it against Astel, the version in the Consecrated Snowfields. 500 damage fully charged if it's a headshot, ~300 IIRC if it's not. And I rarely had the luxury of charging it fully, so those numbers are even lower. Dude has over 18k HP, would've taken forever. I ultimately beat him with Black Flame, the percentage-based bonus damage seemed much more efficient.
Yeah, the lightning spear works best in tandem with a regular attack. It's basically a magic bow. Go in for the attack on Astel, retreat when she starts doing AoE's, shoot lightning from a distance, get close again, etc. No offensive incant will do the job on its own.