Definitely start keeping a spray bottle (or 2 or 3) around and handy. A cat scratch, or worse a bite, can become serious quickly; in 15 years I have twice had to be hospitalized by a bite from my own cat. A cat in a frenzy attacks anything moving nearby, not distinguishing cat from human from HuMom.

In my experience, nothing breaks up a cat squabble faster than a squirt of water.

I agree with Donna, the stray must have marked the firewood so the intruder is constantly present. Wood piles are a safe place for outdoor cats to huddle and warm themselves, so this is highly likely.

Although I have not used the collars, I know several on here who have had success. Give it a try!

And if I remember right, no one was thrilled with the Feliway plug in, they all had more success with the spray version.