For really stubbon pedals I place the spanner in such a position that it makes an accute angle to the crank, in the right direction, and then squeeze the crank and spanner together like a nutcracker. The spanner only needs to move a small distance and so trapping or gouging of fingers isn't an issue.
If it is still beyond even my hand strength I set the spanner and crank as before but with the pedal roughly horizontal and the spanner pointing slightly upwards. I place a solid object under the pedal end of the crank, a lump of wood or a car jack set to the right height works well bit other solid objects will do, and then I hit the end of the spanner downwards with a hammer.
That one never fails and is safe if you take a few precutions.
Hold the pedal still keeping your hand clear of the hammer and spanner.
If the spanner won't stay still hold the shaft of the spanner loosely from above while hitting the end of it downwards. That way if it suddenly goes free it will slip out of your fingers and not take them into the crank.
Having the spanner travel across the BB axis reduces the turning effect on the crank. Having the spanner pointing away from the BB, in line with the crank, requires a lot more effort being spent preventing the crank turning or lifting the bike and so less of the available effort goes into loosening the pedal.
