Um, I don't see the problem here. You said yourself he's easy. He was easy for me too. And I'm sure he was easy for a lot of other people too. So what is there to complain about?
This Giant Beetle Stagnox? C'mon. He is one of the easiest bosses in the whole series. This Fraaz guy, the second boss was much harder. And he killed me once at the second challenge in Hyrule castle.
...? Are you talking about the Take 'Em all on challenge? 'cause every Temple boss is refought in there. You also have to remember that it's a mini game, so it's not actually part of the story. If you're complaining about it being too easy, be glad it's not too hard.
Huh? What's the issue with having an optional minigame allowing you to refight the boss of a dungeon? It's a common opinion that Zelda games should have a way for you to refight bosses, because really they're one of the most annoying aspects of the name to find a way to re-experience. You'd have to play through the entire game again to fight them another time. Having an optional means to fight them again (with a reward for doing so) is a good game design element. I also don't exactly see why you're complaining about Stagnox specifically and not the rest of the bosses.
I like to rebattle them and experience the fights again and relive the moment. Maybe Nostalgia wants me, or I love to battle and own bosses as a trophy of some sorts.