So a unit with 4/4/4 and AC1/1/1 using armor piercing?

p175 says when using armor piercing, you must allocate two of the attack rolls to it and then roll one attack roll.

So you would take 2 of the 4 base damage for your armor piercing, and roll 1 as armor piercing (+1 target number, 1 damage and doing a crit on 10+) and then the two remaining as standard attack rolls.

If the 4/4/4 unit had AC2/2/2 using armor piercing, it could

1) use all 4 and no armor piercing

2) use 2 for armor piercing and 2 without.

3) use all 4 for armor piercing and only roll two attack rolls.

The minimal damage ruling on armor piercing would only come up with a 1/1/1 unit with AC1/1/1. Since it doesn't have 2 damage to allocate to armor piercing, it would be 0* when using armor piercing.