It sounds like you're not saying that it's impossible to heal and slaughter effectively at the same time, but just that it's very difficult to slaughter effectively on a divine caster unless you have some really top notch gear or a few past lives (or both) under your belt.
If so, then I'm inclined to agree with you. It's a lot easier to get a decent DC on a Wizard than it is to get a decent DC on a Cleric or FvS. It's one of the reasons why I went melee + healing on my FvS instead of offensive casting + healing. Way less investment required to make the character perform well.