Yeah, I managed to do this one one of my bikes once. There was no way I could brute force the chain from between the cassette and the spokes.
My suggestion would be to use a vice, or do you have one of those Workmate things?
The first thing I'd do is to break the chain and get the whole "wheel and chain" mess away from the frame. It just gives you more room to work. Then, I'd clamp the teeth of the cog in the vice (as I say, I've had good results with my Workmate). Then you undo the lockring, which admittedly can be fiddly. If it has been on a while it will be tight, as you have discovered, and you might need to use a mallet to coax it loose. (I have tried chain whips in the past too, but never even bother nowadays, a vice is far more efficient.)
If you don't have any way of clamping the cog, the only other suggestion is to take the wheel to a shop, who will use their vice to loosen it. With the right tools, it is really only a 30-second job so you may be able to sweettalk them into doing it for free.
Lastly, you should probably spend some time thinking about how this happened. Could be you just got a jump because the chain was loose, but it is probably worth checking your chainline to satisfy yourself that everything is ok.
You might also want to check your spokes for any damage from the chain.....better to replace them now, while you've taken things apart, than have them break later.