Haven't had much success with fire myself, although I only tried it once. Pretty much the same observations, there- I burned through stamina pretty quickly, and the healing seemed mediocre even if it was AoE.
The heart's primary lightning abilities are actually pretty reliable, if you're using them in the right situations. There's one that thunderbolts 1-3 enemies at random, with a 70% chance of hitting each and a chance to shock, and another that restores a bunch of SP when you shock. It's pretty unreliable versus one enemy, but use it in the general vicinity of a group and you can crank out quite a bit of damage without using much SP.
I usually grab a second ranged attack for when I need actual reliability, preferably one that can't miss. The lightning/light combo I mentioned above is nice because it gives access to censure. 40-some power, but it's ranged, has perfect accuracy, and a lot of things are weak to it.
------------
Headless is weak to lightning, so that's probably why he took damage so quickly. If you're already picking up those decurse healers, they're resistant, and sort of decent at close range if you can actually get them to attack something. Might make okay meatshields, if you don't end up needing the slot for something better.
Vikhor is fast, so the key is basically just finding multiple things that can weather the attacks, then keeping him occupied in melee until you can beat him to death. I don't think he himself has any lightning attacks, so a properly-positioned zombie or headless might be more effective there. If you've got a good way to take out his support before dealing with him, that's probably also a good move, but that tends to be hard if you encounter him at a low level. Dealing with him is definitely hard, and I feel your pain on that particular situation. :/