Took me a second to realize why it was 5 mana. But it doesn’t specify who has to control the fighting creatures. Have your opponents two biggest things fight and kill one another.
The problem with defining this format by what is "fun" is that everyone seems to define fun as what they don't lose to. If you keep losing to easily answered cards, that means you should improve your deck. If you don't want to improve your deck, then you should come to peace with the idea that you are going to lose because you chose to not interact with better strategies.
Clash of Titans 3RR
Instant
Uncommon
Target creature fights another target creature. (Each creature deals damage equal to its power to the other.)
"Let the brutes fight. If they kill each other, we won't lose any more soldiers today."
— Orthion, captain of Lavariva
regularly seems pretty meh, though
"Let them fight."