I once did this for units I wanted to be tougher;
Name them all and have a trigger on ur map, in condition field type true
activation type: egroups = [e1,e2,e3,e4,e5,e6,e7,...];
Make a description.ext with in it:
// AI Skill
//
titleParam1 = "Difficulty:";
valuesParam1[] = {0,1,2};
defValueParam1 = 1;
textsParam1[] = {"Hard","Normal","Easy"};
and then have a script skill.sqs
?(param1==0): goto "hard";
?(param1==1): goto "normal";
?(param1==2): goto "easy";
#hard
{_x setSkill 0.75} forEach egroups;
exit;
#normal
exit;
#easy
{_x setSkill 0.25} forEach egroups;
exit;
edit:
If you want to do this for SP, then don't make the description.ext
Make 2 radio triggers on the map (alpha - bravo)
Condition: this
Activation: [easy] exec "skill.sqs"
or
Condition: this
Activation: [hard] exec "skill.sqs"
and have ur skill.sqs
_skill = _this select 0
? _skill == easy : goto "easy"
? _skill == hard : goto "hard"
#easy
{_x setSkill 0.25} forEach egroups;
exit;
#hard
{_x setSkill 0.75} forEach egroups;
exit;
not using the trigger will make you play the AI with the skill you set them.