All my Pokemon are at lvl 45, the level you encounter Groudon at, and I haven't even beat the Mossdeep City Gym, and Groudon is in a cave in Sootopolis City, by the time I get there, at the rate I'm going, will I kill Primal Groudon?
I do believe they will be too powerful if they're going to be spamming their strongest attacks. Decently powerful neutral Special Attacks should be good. Such examples include Ice Beam on Swampert and Shadow Ball on Gardevoir. Sceptile/Blaziken should be using a weaker STAB move than their main STAB move, like Leaf Blade if you have a Special Sceptile, or Dragon Claw on a special Mega Sceptile, Sky Uppercut on Blaziken (if you manage to obtain it by then).
You can also use the Pokemon damage calculator just to be safe. Remember to save your game before the Primal Groudon battle, so that you can have a chance at catching it again if you defeat it.