Try a manual screw driver but turn it with a fast jerk sometimes that will break them free.
Get a small cold chisel, Use it to create a furrow in the upper side of the screw head. When it looks decent use the chisel and give it a good hit with the hammer. Often they will come loose without the good hit at the end.
If all else fails cut the heads off with a Dremel/moto tool.
It is nice if you can just cut off the part of the screw head that is holding the bracket as this will leave you with a longer stud to grab with pliers and unscrew once the bracket/element out. This is often difficult to do in the confined space of the oven.
Also it is a good idea to clip something (clothespin etc.) onto the wires so that they do not disappear into the back of the oven before you get the new element attached.