It was a struggle. Here's how I overcame:
I picked up a long L-shaped 12mm hex wrench and cut the short end off, leaving a straight 12mm hex. I slipped the uncut end of the hex into the top cap. I then took a 12mm standard wrench and put it over the hex. Using a second wrench, I wedged it into the crescent end of the first, increasing my leverage. I then pushed and pulled with all my might, without much initial success. Once I combined Spanish profanity with brute strength, it finally budged.
In retrospect, I probably didn't need to cut the hex wrench- I could have just slipped the wrench through the hex and then into the top cap, but it made it easier to adjust the wrench position for maximum leverage. By far, getting the top cap off was the most difficult part of the job.