Commander..I wish there was a secret to upgrading a Combines HQ..But theres not..The only way you can protect your HQ above level 5 is by stacking it with Defense,,lots and lots of Defense..There is 1 tip that may be helpful,,Allies..Try to make Allies of the Combines that keep hitting your HQ and downgrading it,, Make good Allies..You can even go into "Allie" status with a strong Combine and they will agree to hit anyone that hits your HQ..If a Combine thinks that they will get hit..or lose lots of units, many will back off