Pierce tells you the minimum amount of damage a character can do in an attack, although a character can never do more damage than its attack total. For example, say a character with Pierce 2 attacks. After flipping battle cards, the attack total is 5 and the defense total is 4. Normally, the character with Pierce would do only 1 damage, but because of Pierce, it does 2 damage instead. This also works if the attack total is equal to or less than the defense total. Say the attacker has an attack total of 5, but this time it runs into a massive defense total of 9. If that character had Pierce 2, it would still do 2 damage. If the character with Pierce 2 had an attack total of only 1, it would do 1 damage against any defense total.
Can other effects change the amount of damage done after Pierce is applied?
[Source: WotC] 
Yes. For example, say a character with Pierce 4 attacks Insecticon Skrapnel in bot mode. (He has the ability "This can't take more than 3 attack damage from a single attack.") After battle cards are flipped, the attacking character would do 2 damage, but Pierce raises that to 4. Insecticon Skrapnel’s ability would then reduce the damage done to 3.
Is Pierce damage still considered attack damage? For example, if someone is attacking me with Pierce 4 against a character like
Insecticon Skrapnel whose text is “can’t take more than 3 damage during an attack”, does that mean even though they have Pierce 4 after adding attack damage, would they only do 3, or would they do 4?
[Source: WotC]
Pierce is an ability that applies to attack damage. But, effects like
Insecticon Skrapnel’s or the
Safeguard ability will still apply. In your example, Skrapnel would take only 3 damage.
Let say I'm attacking with Mirage and flip 2 black pips and 3 white pips. As far as I know, my attack is 5 Pierce 2 and then his ability triggered and I untap. Then I attack again and this time I flip 1 white pip and 3 black pips. Is my attack 5 Pierce 3 or is it 5 Pierce 5?
[Source: WotC]
5 Pierce 5.
If a character’s pierce is reduced to zero pierce would it still be considered pierce? More specifically, since the opposing character is attacking with pierce, but it’s reduced to zero, would Jazz’s ability trigger, allowing only two damage to hit?
[Source: WotC]
A character with Pierce, whose Pierce score is reduced to 0 or less still has Pierce.
Common
Megatron is in Bot mode. He has
Grenade Launcher equipped giving him +4 attack. He attacks Barrage in Bot mode. Megatron does not flip any orange icons and Barrage does not flip any blue icons. Megatron's attack is 8 w/Pierce 3 (due to his ability), while Barrage has 2 defense. Does he only deal 6 damage for the entire attack or 3 only due to his Pierce 3?
[Source: WotC]
6. Pierce is a minimum damage.
Arcee is in Bot mode. She attacks Barrage in Bot mode. Arcee flips 3 orange icons and Barrage flips 2 blue icons. Arcee's attack is 4 w/Pierce 4 (due to her ability), while Barrage has 4 defense. Does she only deal 0 damage for the entire attack or 4 only due to her Pierce 4?
[Source: WotC]
After flipping the three orange, she has Pierce 4, meaning she would do a minimum of 4 damage.
Arcee is in Bot mode. She attacks Barrage in Bot mode. Arcee does not flip any orange icons and Barrage flips 2 blue icons. Arcee's attack is 1 w/Pierce 1 (due to her ability), while Barrage has 4 defense. Does she only deal 0 damage for the entire attack or 1 only due to her Pierce 1?
[Source: WotC]
In this scenario,
Arcee deals 1 damage.
If I have a character with Pierce 2 (for example), and equip it with an upgrade that gives Pierce 4, the Pierce would be the greatest one or the sum of both?
[Source: WotC]
Pierce stacks, so it would be 6.
There's now an upgrade that says "Opponents cannot have more than
Bold 2" or something to that effect. Mechanically, 3 instances of Bold 2 have worked the same as Bold 6. It hasn't been explicitly said that Bold and
Tough stack just like Pierce, so does it?
[Source: WotC]
Yes.
Bold,
Tough, Pierce, and other key words with numerical values do stack.
