Clone is especially useful in the later game, when you have large hordes of creatures at your disposal. Clone can often be considered as the most powerful spell in the game damage-wise, since a cloned creature is likely to do more damage with a single attack than any spell, generally speaking.
Because clones do not withstand any damage, it is advisable to clone a creature stack that has the speed higher than the enemy creature stack next in turn. This is because the cloned creature will be next to act if it has the highest speed of all the remaining creatures.
Good target for clone are creatures with no enemy retaliation, for example nagas and vampires. When this kind of cloned stack attacks, it does not receive counter-attack and therefore does not take any damage and disappear. On the other hand, when fighting against enemy clones, it may be wise to attempt killing the clones first if possible. Monsters with powerful spells or abilities, such as Master Genies or Archangels, can also be good targets for this spell. Interestingly, clones also benefit from morale benefits.