For a fast attack, try the royal guard with 8-10 dewbacks running just ahead of him. Look for rocket farms. That loadout levels them fast. Beware traps and mortars. Burst turrets are also a problem, but you can deal with one or two. The royal guard gets dumped on a lot, but it is amazing how fast he can wipe a base if you lead him with dewbacks. There are usually a ton of rocket farms at that level.
Ditch the atat, maybe use a couple atst's. Bring storms for cleanup. Since you're at level 6, I'd ditch the bikes, maybe take probes. If you have level 6 ava's, use those. At level 6, they are amazing.