Harvesters of Souls is an epic undead unit that adds copies of enemies to your deck when it attacks. The strength of Harvesters of Souls and the strength of the copy it creates can be increased by leveling it up. Harvesters of Souls works well in late-game decks.
Harvesters of Souls' unique ability to copy an enemy card opens up many possibilities. Note that the ability of the copy will depend on the level of your opponent's unit.
Another good strategy is to copy units with 1 strength. They get the biggest boost to their power because their strength gets increased. On the contrary, try not to copy cards like Calming Spirits, as their high strength will be diminished.
Copying cards that are already in your deck can be helpful too. By having two of the same card in your deck, you can draw it more consistently.