Boss Abuse is a way in which Boss Characters are 'abused' to gain high amounts of exp. There are many ways to do this.
The most common method of Boss Abuse is the concept of using a Boss with a constant way of regaining Hit Points to train units. This is most often applicable with Bosses that stand on certain spaces that give them Hit points back at the beginning of their turns, such as the Gate spaces in certain Fire Emblem games.
The player can abuse these situations by having their characters constantly attack the Boss until their Hit Points are low, then allow them to regain Hit Points until it is safe to attack again without killing the Boss. In most conquer-style maps, this can be done infinitely until the player decides to stop. However, this type of training can be very time-consuming.
In the later games the Boss Abuse strategy can also be used on Bosses that have a skill that gives them a set amount of Hit Points back per turn, such as Mantle. This is somewhat less effective, as bosses with these skills generally are stronger or have long lasting weapons.
This strategy is often used to train weaker units of lower levels. This is done by having higher level units battle with the Boss until the Boss's weapon runs out of durability points and breaks, leaving them defenseless as the weaker unit does small amounts of damage with a low cost, high-durability weapon to get lots of experience points. This is possibly the only real use for the Corrosion skill. This strategy also works best on Bosses with less weapons or lower-durability ones, so that it takes less time to have their weapons break.