Artillery is a support units, so in battle artillery elements are put in second line and take very few damage as long as there is still frontline troops (infantry to covert them).
It think it do not really matter where you put artillery in the stack. The model 2 divisions + 1 or 2 medium arty for corps is very good.
I notice that for frontline troops, corps take overall fewer damage than isolated units. But it is not true for support units.
So you want your frontline troops in corps but artillery can be anywhere.
If i have generals with artillery special ability, i sometimes make a 100% arty corps. The behaviour is not changed, arty hardly take a hit in combat, if you are not too much on the loosing side.
Heavy arty is overall better in combat than medium but is soo much expansive. The stats to look at is firing rate, power and range.
Assaut combat : Each round of combat in Ageod games have a Fire sequence, then a Melee/assaut sequence.
If you have a battle with 3 round of combat, you had 3 fire sequences and 3 melee sequences.
If you play AJE in roman time, the fire sequence hardly do any damage and the melee sequence is the most important.
In End of all war, the battle is won or lost mostly with fire sequences.
In most Ageod game you see how much damage have been done in melee and in fire sequences, but in End of all war, the battle report have been reformated and everything is merged.