RMVX has animations for that, all you need to do is compose them. They have ones preset for fire, ice, etc... sure, but if you look at them you'll notice that you can use two separate animation sheets - one is the blow animation; the other is the element animation. For the slash ones, they do have the elements directly in the animation, but that is by no means necessary.
All you need to do to make a Slash/Darkness animation, for instance, is to make a new slot, choose the Slash animation as the first graphic and choose one of the darkness animations as the second graphic. Then compose the animation. You don't need new resources for this request, you just need to learn how to use the ones you have a little bit better.