Easy way to remember which pedal screws in which way-
"RIGHT IS RIGHT" – the right hand pedal screws in out the CORRECT way you'd expect i.e. it goes in clockwise and comes out ant-clockwise. Left pedal vice versa.
Most pedals (with flats) fit a 15mm spanner so if you can get one with a good fit and slip either a bigger ring spanner over it or some tubing of some kind to extend leverage then go for it.
If still too tight, alloy expands more than steel as it heats up so put the end of the crank arm over a camping stove or blowtorch etc for a little while prior to trying the pedal again.
Recently had to heat a crank for ages then slip a 6ft marquee pole over the spanner and still needed huge effort to undo the pedal – but it came off with no damage.