Mage Wars' Rulebook v2.0 states on p.22 under "Step 1: Declare Attack" :
[...]
Then, announce what target you are attacking. You must choose a legal target for your attack:
If you are making a melee attack, the target must be in the
same zone as the attacker. If there are enemy guards in the
zone, you must choose one of the guards to attack (see
“Guarding” on page 29).
So, I understand it the way that the attacker chooses which guarding creature is attacked.