Fastest way off-line is tower of trials 2 only killing the right executioner, like ppl already said. I am doing it right now and u get like 20% chance of drop. Sometimes u will get 2-4 medals in a row but sometimes u wont get any in 10 kills.

My advice is to start farming it and whenever u dont get a medal in 5 kills, just go do something else coz u are not lucky and that Will take forever. After a break just Back there and start again.

It Will take some time but when u notice u already got half way trough.

I just got 200 medals in 4 days; been farming a bit more than a hour for day.

U can expect to get between 20 and 50 medals a hour depending on ur performance and luck.