BTW, my HQ's at level 7.
I just started trying out the Mobile Heavy Cannon after unlocking it a couple of days ago. Before that, I've been using 13 Dark Troopers, 30~ Stormtroopers and an Elite AT-AT to attack, which proved to be very effective ONLY with buffs like the AT-ACT and the Shoretroopers.
I'm currently using 4 Mobile Heavy Cannons, 2 Repair Droids, 20-30~ Stormtroopers and an Elite AT-AT, no buffs included as I relocated to Hoth. I've never gotten 3-stars with the Mobile Heavy Cannons. They would just be destroyed early with the Elite AT-AT before even getting a star or run out of time due to their insanely low movement speed.
Does anyone have any useful attack strategies for HQ 7 that can 3-star almost all the time? I would love to hear them
Also, any videos to support your claim would be greatly appreciated!