2002 Chevrolet Cavalier Just Bought This Car M Trying Change Spark Plugs S

I just bought this car and I'm trying to change the spark plugs. It's a 2.2L ECO-TECH. A previous owner must have tightened them as tight as possible cause I can't seem to break them loose. I got one to come out, but I'm afraid to use too much force thinking I may twist one off, or ruin the threads in the aluminum head. I know that the engine must be cold to take them out. Is there any tricks to this I could try, or should I just use the force needed to break them loose? I did think if I used my impact wrench, this may help to vibrate them loose. Any suggestions will be greatly appreciated.