No good suggestions from me. It can be a challenge. I have been using power pellets (Velpar) with some degree of success but on the big trees it can take a couple years. Cutting and treating the stump may be much better.
I checked a few this weekend that I treated with the pellets over a year ago. A few are dead, some you can't hardly tell they were treated, but most are slowly dieing. I went ahead and threw out some more pellets even though this is not the time to put them out. The directions say not recommended for clay soils and that is what this is so I guess I shouldn't be expecting miracles. Tordon leaf spray seems to work the best on the small bush type trees.