Level 5 is when Rebels first get Burst Turrets, so as you may have noticed, you can't solo their base with the Royal Guard anymore haha. It took me a while to figure that out when I first hit HQ 5. Here's the loadout that made a huge difference for me and got me back to 3 starring almost everyone I came across-
1x Elite AT-AT
I think those were the right amounts... Been awhile so I can't remember the exact capacity of your transports, if it's more or less just scale it down/up. But yea, drop the AT-AT near your entry point (preferably near a shield gen) and then drop all those giant lizards and watch em tear their shit down. They will break straight through all the walls and rush the turrets. And there are way too many of them for most turret combos to even make a difference. Drop the stormies after the lizards break in and aggro the turrets for extra damage and to clean up buildings.
This combo works up through HQ 5 and most of 6. The only bases I had trouble with were ones intentionally designed to drag troops all around the map, under fire and with well-placed traps or ones that just had a shit ton of mortars.
Hope this helps!
Oh, also, if your entry point has a bunch of rocket turrets within range of where you're going to drop the eAT-AT then drop the dewbacks first.