I know its a noob question, but the last Pokemon game was Gold, so coming back to HeartGold is really awesome. So is Battle Frontier something you need WiFi to play?
No. You don't need WiFi to play in the battle Frontier. You only need WiFi to battle other real people on the battle frontier.
No, you can use the Battle Frontier without a wi-fi connection. There are some parts where you can battle online but most of it is "offline".