S4. Find out the target’s lowest armour value
So that you will know what you need to roll to hit (and which dice to discard as misses), check what is the lowest Armour value in the target detachment.
Remember that infantry in cover benefits from a +1 bonus to its Armour value (or +2 in fortifications).
You target an Ork target detachment that contains: Gretchin (Armour 3); Gretchin that are in cover in some ruins (effectively Armour 4); Ork Boyz (Armour 4); Battlewagons (Armour 5).
Any die you roll that scores a 3 or more will be hit. Any die you roll that scores a 1 or 2 will be a miss.