Get a guaranteed open end wrench (Harbor Freight now guarantees them).
Impact works wonders.
put crank in a vice if you can, make it pretty much immovable.
if you get desperate, stick a large screwdriver thru the chainring, turn chainring until screwdriver is lying on bike frame.
Have someone hold the screwdriver if you have help.
if the frame is nice, wrap screwdriver in shop rag where it contacts the frame.
I usually to do this with the chain still on, and hit it in the backpedal direction like you are applying the brakes (assuming a coaster brake).
put wrench on pedal, hold wrench with pliers, hit wrench sharply with a hammer, Brass hammer if you have one.
don't hit your thumb!
Remember :
pedal on right side (chainring side), lefty-loosey -- righty-tighty. counterclockwise to remove.
pedal on left side, just the opposite righty-loosey -- lefty-tighty. clockwise to remove.
If you screw up wrench, take it back for a new one under guarantee. (you won't).