Spontaneous castings are 1. so random in effect and 2. considerably lower in power than spells you can cast unless you're willing to accept a lot of shifts.
If you up your skill level in what you want to spontaneously cast, you should be fairly safe. You can also work it out so that even with shifts, you can pretty much get what you want done. Just a few checks with mine and I get this:
spontaneous magic (Harm/Death - 5-2-1) (2 shifts)
spontaneous magic (Harm/Death - 3-2-2) (2 shifts)
So with just a bit of overcasting and some creative descriptions about what you want to happen, you should be able to kinda get what you want. So if I wanted to seriously wound my opponent I might go for the first one instead so that the worst that can happen is that effect is brought down to a 3. Best that can happen is my opponent has a very seriously wounded hand that is gradually wounded over an hour.
Of course you'd have to describe what your character wants very well so that the person picking it up has something to work with. Plus you have to hope they listen to your character's intentions at least a little bit. But using only spontaneous spells is entirely possible.
