How to get out extremely difficult spark plugs?

My car is a 2006 Honda Civic. It's 1.8 liter
I changed 2 spark plugs but the other 2 absolutely can't get out. I had a guy friend whose pretty built try to remove it but he almost pulled his should trying. I need to know if there's anything I can use to get it out. I also don't know if it's possibly cross threaded. It's a used car.

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