Wow, a mess is an understatement. With no claims that it will work without damage to the gun, try this.
Look into the hole where the takedown lever was and you should see a round hole with a kind of keyhole at the front. Insert a small screwdriver and see if you can turn the part the keyhole is in. It is the stop lug that prevents the slide from coming back off the frame when firing. The takedown lever moves it forward and down out of the way. There is a lug on the takedown lever that works the same as the screwdriver to do that. If you can get that lug down and hold it there, you MIGHT be able to move the slide back off the frame. But with the lug in the upward position, there is no way the slide can come off short of a cutting torch.
There might be another way. At the front of the slide there is a small pin, in a vertical position. If you can find a good machinist with a good drill press or milling machine, he might be able to use a stiff starter drill to drill that pin out. (It can't be driven out as there is no place to drive it with the slide in place.) That will free up the plug that supports the recoil spring and let the spring come out the front of the slide. The spring will be ruined, but that is a given anyway. Then maybe the rest of the recoil spring stuff will fall out through the mag well. Then do the thing with the screwdriver in the takedown lever hole.
If all else fails, see if you can buy a new slide and takedown lever. GPC lists the slides at around $80 complete, $50 stripped (buy the complete), the levers at about $30. Then, with new parts in hand, use a cutting wheel to just cut the slide off.
You will probably find you need a few small parts as well, but you can deal with that when you need to.
It is a drastic solution, but better than ruining the frame by trying to beat the slide off.
Jim