Sadly, this was one of those episodes where the story was more than the special effects and costume budgets of the time could plausibly portray. The original fight scene was hilarious by modern day standards, though at the time I suspect it was about the best they could do. After all, they were trying to have a personal combat between Kirk and a creature far stronger than he was that lasted more than five seconds and didn't end with Kirk a puddle of goo on the ground. Quite the dramatic quandary.
The story of that episode, "Arena", at least as it was presented in the James Blish adaptation that I remember vividly to this day, was one that impressed me by its ambitions. The idea that Kirk, as a military commander, would spare an enemy when he had the chance to have them destroyed, and preferred trying to negotiate with them when they'd savagely attacked a Federation outpost was a thought nearly as foreign to most Americans back then as it seems to be today. Gene Roddenberry, who was a World War II veteran, perhaps understood the implications of both war and negotiation better than most people in this day of professional expeditionary forces. When a President is applauded for not negotiating with a country that isn't actually harming us but is doing something we don't approve of, as George W. Bush was with Iran, then I think maybe there are a few people who ought to watch the end of that episode and think about what real courage in the face of an enemy is. Often in war, being humane is the toughest thing to do.
Afterword: Believe it or not, Mythbusters looked into the plausibility of the "Gorn cannon" from "Arena".