When I had bad endo cramps, I took naproxen as soon as I got my period and for the first 2 days afterwards. Don't wait for the pain to start and then respond, it's already got hold by then - as soon as you see the first sight of blood, take the NSAID and keep taking it regularly for the first 2 days (or whenever you get the pain).
I used to prefer Naproxen to ibuprofen because it seemed to be more effective and one capsule of it lasted 8 hours, so I just needed 2 capsules (Am and PM) each day. However I think it is prescription only in the UK now. In the US you can get it over the counter, should you ever take a trip there.
Other ideas include a TENs machine you can get for period pain which you put on your stomach under your clothes. I haven't tried that.
But really you probably want to get some things on board to stop the pain systemically rather than just reacting to it each month. Get a copy of Lara Briden's 'Hormone Repair Manual' as she has lots of suggestions. For endo, she recommends NAC and berberine and zinc... and magnesium. I'm on everything she recommends and I have a history of endo. I don't have any pains at the mo although I'm on 200mg of utrogestan continuously which helps suppress the endo. You could ask about continuous progesterone too, if you are on HRT...