This card has relatively weak stats for its price, however its ability to swap the ATK and HP of any Board Card makes it a recommended addition to most decks. This can be used on your own cards or on the enemies.
When used on enemies: Madjick is particularly useful against enemies with low ATK and high HP like Pyrope, Igloo, Memorial Statue or Gaster in order to instantly kill them (0 ATK) or able to kill them faster (1 ATK), especially with the passive ability of Justice. It can also be used the other way around on monsters with high ATK but low HP, such as Mad Dummy or Mettaton NEO, in order to make the next attack less painful or to finish them with Muffet's Pet.
When used on allies: Here, Madjick is also mostly used on low ATK, high HP monster, so they turn into what is known as a Glass Cannon. A popular use of this is with Flowey, as not only will it get an ATK of 6, his ability will make him keep his ATK buffs upon death. This makes him turn into a 9/6 monster after just 1 use. Madjick can also be used to lower the ATK of your monsters. This is mostly done in order to give them higher HP to survive the next attack. This tactic is only really useful on most Legendaries and DT's, as they have both the ATK and HP needed to survive an attack and to gain a lot back when swapping its stats.