Piercing attack

Piercing attacks are damage-based attacks in the Mario RPG-based games that ignore all or almost all defensive capabilities of the opponent. These attacks are very useful when dealing with a highly damage-resistant enemy, but can be detrimental when used by enemies against a defense-heavy Mario. These types of attacks are seen in most of the RPG-type games.

Super Mario RPG
There are no damage-based attacks or magic attacks in this game that ignore Defense or Mg. Defense, but there are a few items which do a fixed amount of damage to any enemy regardless of defense:

Fire Bomb

Ice Bomb

Rock Candy

Star Egg

The Geno Whirl also does a fixed 9999 damage to all non-boss enemies except for Exor.

Paper Mario
In this game (and the following one), attacks subtract directly from defense, which means that highly-defended enemies can be taken down easily with these kinds of attacks. All damage-based items (such as Fire Flower and Thunder Rage) pierce defense by default, and the special attack Star Storm does as well. Other than those, these attacks pierce enemy defense as well:

D-Down Jump

D-Down Pound

Quake Hammer

Super Quake

Mega Quake

Watt's basic electric attack

There are no known enemy attacks in this game that can pierce Mario's Guard abilities or badges.

Paper Mario - The Thousand Year Door
There are many abilities in this game, both friendly and enemy, that can pierce defenses. This is a summary:

Mario + Partner
As in the previous game, all damage-based item attacks are immune to enemy defense, as well as special attacks such as Earth Tremor and Art Attack. There are also a few normal attacks, such as:

Piercing Blow

Yoshi Kid's Gulp

Ms. Mowz's Slap

Note: There are NO defense-piercing abilities short of the Yoshi Kid's that work on the Armored Harriers, not even the Superguard.

Enemies
Unlike Paper Mario, some enemies in this game do have defense-piercing abilities. Those that do will ignore all Defense Plus and Damage Dodge badges that Mario or his partner have equipped. D-Up, P-Down and D-Down, P-Up badges, however, are not affected, allowing even defense-piercing enemies to do more or less damage than otherwise expected. Also, since enemies can use items in this game, their defense-piercing properties still hold when used against against Mario.

Even piercing attacks are not unblockable, however. Mario's Guard will still reduce the damage by 1, and the Superguard will prevent all damage, as it always does.

These enemies include:

Yux

Z-Yux

X-Yux (immobilization effect even pierces Guards)

Dark Wizzerd (beam attack)

Wizzerd (beam attack)

Elite Wizzerd (beam attack)

Shadow Queen's hands