The weakness when you use the HQ alone is they can run ahead of your main force and occupy enemy territory then get crush by reinforcement

. HQ alone can also take huge damage by airforce. The last thing is if you don't have skill to micro your force (like me

o) then suddenly you found all your HQ run into 1 provide :rofl:! So the better way is attach some division to them to make them have the same speed of your main force
I.e:Your main army is inf so attach inf with your HQ. Your main army is mobile then attach 2 armor or Mec to protect it (and have 5% bonus too).
I often deploy my army into: Single division,Small,Medium,Large and Grand Corps
_Single division often use as scout. Attack first then retreat to lure the enemy and spot how many they have. Cavalry is the good one because they are cheap and quick to replace or use to suppression (Gar/pol)
_Small Corps with 3 divisions. they're your workhorse often 2 inf/art-1 mil use to guard important cities or as frontline garrison and less mobile support force. The mobile Small Corps of armor-mot or 3 cavalry divisions can also use to capture unguard (or very weak) provide. Sometime (especially in Historical scenario) I put 2 inf-armor to have some extra bonus
_Medium Corps with 9 divisions of inf/art use to hold the ground or attack. They can also a mobile corps of Armor-Mot-Mec use to punch through the enemy line and make pockets or act as quick reinforcement units.
_Large Corps with 12 divisions act as Medium Corps but use again more solid enemy line. However i'm not use them quite often since Grand corps handle their role far better
_Grand Corps (or Army) the cream of the army consits of 24 divisions

. They always have 3 smaller corps. 2 first have 9 divisions under the command of Generals (i'm not good in control too many corps so i prefer to put them together under the leadership of generals or FM) and the third corp have 1 HQ and 5 more divisions under the command of FM. This force often consits of pure inf/art or more stronger Mtn/art. This kind of army put the use of HQ to maximum effect they will strike down the enemy with their huge firepower and number. They also flexible and strong enough to quickly divide and attack many medium and weak enemy position
PS:Attach MP to your HQ to get 3 more Org