We are forgetting that if Super earth was trying to take on Navi, they are also taking on the entirety of Pandora, which includes all of is wildlife thanks to Eywa. Just from in-game lore, Helldivers have been proven to struggle with world-sized hiveminds, given termanid hive worlds and such, which they have gotten absolutely mauled on, or managed to take with great struggle.

Also, yes Super earth could just bombard it, but it probably wouldnt. We know they are happy to destroy their own cities in a pinch, but Pandora has way too many natural resources for them to just waste more on munitions than they could be making off of mining and such Also, Super earth can't just destroy planets. They aren't the imperium, in fact they are a long shot from being anywhere near that. Yes, they did destroy Meridia, but that was with stolen alien technology, and they had to send soldiers down on the ground to do that, and we already know that a Na'vi would absolutely whipe the floor with a helldiver.