A First Strike is a term to describe someone attacking an enemy before the battle begins. This term is first seen in Paper Mario and is mainly seen in most of the other Mario RPG games. Note that some enemies can also get First Strike on players (such as Koopa Troopas).
First Strikes are first seen in Paper Mario. After Mario defeats Jr. Troopa and heads right back to Goomba Village, Goompa tells Mario about First Strikes and how they are helpful in enemy fights. During the course of the game, Mario himself can use his Jump or Hammer abilites to get a First Strike on enemies. It should be noted that using the Jump as a first strike wouldn't work on Spiked enemies or Fire enemies (unless he has the Spike Shield or Ice Power badge equipped). Also, partners such as Kooper and Bombette can also use their field powers to gain First Strikes on enemies. Kooper's ability is helpful for attacking enemies from a distance and Bombette can flip enemies like Clefts for a helpful lead. It should also be noted that enemies like Koopa Troopas, Monty Moles, and Hammer Bros. can also get First Strikes by attacking and most times, it can be pretty difficult to Guard against. Note, Mario cannot use the Action Command when he gets First Strike.
In Paper Mario: The Thousand Year Door, First Strikes return and are pretty useful. Mario can now use the Action Command and can even follow up with Stylish Moves. First Strikes in this game has the small chance of bringing the crowd to their seats.
In Paper Mario: Sticker Star, First Strikes serve their purpose as before damage to enemies. As Mario fights alone in all of the fights, it is highly recommended to get First Strikes against enemies as much as possible. Mario can also use his Hammer to use First Strikes.
First Strikes are a little different in this RPG series. In Mario & Luigi: Superstar Saga, Mario and Luigi can gain First Strikes by either jumping or hammering enemies. However, when a First Strike is obtained, all enemies on the screen are damaged. Enemies have a unique way of getting First Strikes. If an enemy manages to move in on one of the Mario Bros. from behind, Mario or Luigi will be shocked, say "Oh no!", and will be unable to attack on that turn. However, this is negated if Mario or Luigi is faster than the enemy.
In Mario & Luigi: Partners in Time, First Strikes are basically the same except the action for the Hammer. Whenever a First Strike is used with a Hammer, instead of damaging the enemy, the enemy is temporary stunned leaving it wide open to an attack.
First Strikes return in Mario & Luigi: Bowser's Inside Story with the same properties from the first game. Bowser can also receive First Strikes by either punching enemies or breathing fire at them from a safe distance.
First Strikes return in Mario & Luigi: Dream Team. Whenever Mario or Luigi gets a First Strike against an enemy either by jumping or hammering them, the screen turns over to a symbol of the used attack and the color mainly displays which Mario Bro. attacked. For example, if Mario hammers an enemy, the screen will turn mainly red with a giant picture of a hammer in the middle. If an enemy gets to Mario or Luigi first from behind, the screen turns purple with a large yellow star in the middle.
Actually, The Goomba Bros., the first team fought in the Glitz Pit in Paper Mario: The Thousand Year Door, are the only enemies in the series that can get a First Strike against Mario since it was part of the storyline. Skill players are able to avoid First Strikes from any other enemies in the series.