Buddy nut or a stud removal tool is your best bet, cutting a slot for a flat head never worked very well for me and wound up ruining my chances of getting a nut to thread on.

There's a great product out on the market that outperforms liquid wrench/PB blaster/mouse milk/etc., it's called Bolt-Off from Nutek.
I did a side-by-side comparison of those three and the Bolt-Off by spraying a different one on axle U-bolts on a truck (4 nuts, 4 penetrating oils, and the other side of the axle was used as a control group by repeating the experiment).
Compared to the Bolt-Off, the rest of the nuts acted like they were dry, creaking and popping, and needed a wrench almost the whole way.
The Bolt-Off made a single pop when the torque let loose, required a wrench for 2 turns, then was handy the rest of the way.
Once I got to the other side and found it to be the same, I sprayed the Bolt-Off on the other nuts and they came off without any complaints.

That really made a believer out of me, and I highly recommend it.
http://www.amazon.com/Nutek-Green-Bolt-Penetrant-Lubricant/dp/B002TNX3GY