If it really is creosote, it should burn very easily if you leave the top off. In fact, creosote is what causes chimney fires. It is so volatile, once you get it ignited at the bottom, it should burn right up to the top by itself leaving a surprisingly thick ash which easily brushes off.
Jim - Another transplanted Texan
Former KCBS CBJ
Large and Medium Big Green Eggs , Black 18.5" WSM, Blue Weber Performer - Stainless, Green Weber OTG Kettle , Brinkmann SnP Pro, and a Stainless UDS. One retired Portable Kitchen grill.
Red Thermapen, Maverick ET-732, EdgePro Apex Sharpener.
Avatar is the original 1951 Weber Kettle