This is an opportunity to try my engineering and ordnance experience. It is NOT based on any experience with trying to pull a stuck pump. Some of this you may consider humorous.
1. Pump some hydrochloric and/or sulfuric acid down into the vicinity of the pump to dissolve some rock or rust to loosen it up.
2. If it has a steel drop pipe or cable to pull on, you could hook up a puller with a hydraulic jack that would jack against the casing. You could easily apply as much force as the pump will stand. I can give you a puller design if you need it.
3. If not a steel pipe or cable, rig something to get the plastic out of the way and screw a rod with a special fitting into the pipe fitting on the top of the pump. A special pipe fitting could be made with extra taper to make it easier to engage the threads. Then pull as per above.
4. If a steel pipe is available, can you altenately pound and pull on it to kind of work it back and forth.
5. If there is space below the aquifer, can you break off the pipe and just abandon it down the hole by pounding it below the aquifer?
6. Seal off the casing around the down pipe and put an explosive mixture of propane and oxygen down the down pipe to create a bubble below the pump. Using a wire that you have fished down the pipe before you pump down the propane, apply enough current to ignite the mixture. ALL OF THIS SHOULD BE DONE BY REMOTE CONTROL, AND NOT NEAR A RESIDENTIAL AREA.