Fentanyl citrate is slightly soluble in ether. MDMA hydrochloride is insoluble in ether. You can just wash your MDMA with ether multiple times and it will wash away the fentanyl. Ether boils at approximately 98.2° which means it'll evaporate at room temperature pretty easily. Or you can use a hair dryer on low from across the tent, you don't want either fumes being ignited by the element in the hair dryer.
Slightly soluble isn't really an issue because you're going to use enough ether to completely soak your MDMA, and any few milligrams of fentanyl citrate will easily be dissolved. Wash it two or three times, and let it sit in the ether for a while. The MDMA will not dissolve. However, cut, fentanyl, and things you thought were MDMA, but maybe weren't might dissolve as well.
Oh yeah, multiple coffee filters so you don't lose your MDMA.
I don't know if the fentanyl test strips will be bothered by the ether or not. It'll be a shame to wash the MDMA and not test the leftover ether.
Easy peasy. Lemon squeezy.